Ciao a tutti visto che il mio computer continua a spegnersi allego il log:

Logfile of Trend Micro HijackThis v2.0.4
Scan saved at 19.00.53, on 01/02/2012
Platform: Windows Vista (WinNT 6.00.1904)
MSIE: Internet Explorer v7.00 (7.00.6000.16982)
Boot mode: Normal

Running processes:
C:\Windows\system32\Dwm.exe
C:\Windows\Explorer.EXE
C:\Program Files\Windows Defender\MSASCui.exe
C:\Program Files\Synaptics\SynTP\SynTPStart.exe
C:\Acer\Empowering Technology\eAudio\eAudio.exe
C:\Windows\System32\rundll32.exe
C:\Windows\RtHDVCpl.exe
C:\Program Files\Intel\Intel Matrix Storage Manager\IAAnotif.exe
C:\Windows\system32\taskeng.exe
C:\Windows\System32\rundll32.exe
C:\Users\marco\AppData\Local\Temp\RtkBtMnt.exe
C:\Program Files\Synaptics\SynTP\SynTPEnh.exe
C:\Program Files\Launch Manager\QtZgAcer.EXE
C:\Program Files\Acer Arcade Deluxe\Play Movie\PMVService.exe
C:\Program Files\Alice ti aiuta\McciTrayApp.exe
C:\Program Files\Common Files\Symantec Shared\PIF\{B8E1DD85-8582-4c61-B58F-2F227FCA9A08}\PIFSvc.exe
C:\Program Files\AVAST Software\Avast\AvastUI.exe
C:\Program Files\iTunes\iTunesHelper.exe
C:\Program Files\Common Files\Java\Java Update\jusched.exe
C:\Program Files\Windows Sidebar\sidebar.exe
C:\Program Files\DAEMON Tools Lite\DTLite.exe
C:\Program Files\Acer\Acer VCM\AcerVCM.exe
C:\Acer\Empowering Technology\eRecovery\ERAGENT.EXE
C:\Program Files\Windows Sidebar\sidebar.exe
C:\Program Files\Acer\Acer VCM\acp2HID.exe
C:\Windows\system32\wuauclt.exe
C:\Program Files\Google\Chrome\Application\chrome.exe
C:\Program Files\Google\Chrome\Application\chrome.exe
C:\Program Files\Google\Chrome\Application\chrome.exe
C:\Program Files\Google\Chrome\Application\chrome.exe
C:\Windows\system32\rundll32.exe
C:\Program Files\Google\Chrome\Application\chrome.exe
C:\Program Files\Google\Chrome\Application\chrome.exe
C:\Program Files\Google\Chrome\Application\chrome.exe
C:\Users\marco\Downloads\HiJackThis.exe

R1 - HKCU\Software\Microsoft\Internet Explorer\Main,Search Page = http://it.rd.yahoo.com/customize/ycomp/defaults/sp/*http://it.yahoo.com
R0 - HKCU\Software\Microsoft\Internet Explorer\Main,Start Page = http://startsear.ch/?aff=2&cf=bb479369-3bc8-11e1-b2da-001b24f7ba05
R1 - HKLM\Software\Microsoft\Internet Explorer\Main,Default_Page_URL = http://it.intl.acer.yahoo.com
R1 - HKLM\Software\Microsoft\Internet Explorer\Main,Default_Search_URL = http://go.microsoft.com/fwlink/?LinkId=54896
R1 - HKLM\Software\Microsoft\Internet Explorer\Main,Search Page = http://go.microsoft.com/fwlink/?LinkId=54896
R0 - HKLM\Software\Microsoft\Internet Explorer\Main,Start Page = http://startsear.ch/?aff=2&cf=bb479369-3bc8-11e1-b2da-001b24f7ba05
R0 - HKLM\Software\Microsoft\Internet Explorer\Search,SearchAssistant =
R0 - HKLM\Software\Microsoft\Internet Explorer\Search,CustomizeSearch =
R1 - HKCU\Software\Microsoft\Internet Explorer\SearchURL,(Default) = http://it.rd.yahoo.com/customize/ycomp/defaults/su/*http://it.yahoo.com
R1 - HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings,ProxyOverride = *.local
R0 - HKCU\Software\Microsoft\Internet Explorer\Toolbar,LinksFolderName =
R3 - URLSearchHook: Yahoo! Toolbar con blocco Pop-Up - {EF99BD32-C1FB-11D2-892F-0090271D4F88} - C:\Program Files\Yahoo!\Companion\Installs\cpn\yt.dll
O1 - Hosts: ::1 localhost
O2 - BHO: Yahoo! Toolbar Helper - {02478D38-C3F9-4EFB-9B51-7695ECA05670} - C:\Program Files\Yahoo!\Companion\Installs\cpn\yt.dll
O2 - BHO: Adobe PDF Reader Link Helper - {06849E9F-C8D7-4D59-B87D-784B7D6BE0B3} - C:\Program Files\Common Files\Adobe\Acrobat\ActiveX\AcroIEHelper.dll
O2 - BHO: IE5BarLauncherBHO Class - {78F3A323-798E-4AEA-9A57-88F4B05FD5DD} - C:\Program Files\StartSearch plugin\ssBarLcher.dll
O2 - BHO: (no name) - {83A2F9B1-01A2-4AA5-87D1-45B6B8505E96} - (no file)
O2 - BHO: avast! WebRep - {8E5E2654-AD2D-48bf-AC2D-D17F00898D06} - C:\Program Files\AVAST Software\Avast\aswWebRepIE.dll
O2 - BHO: Java(tm) Plug-In 2 SSV Helper - {DBC80044-A445-435b-BC74-9C25C1C588A9} - C:\Program Files\Java\jre6\bin\jp2ssv.dll
O3 - Toolbar: Yahoo! Toolbar con blocco Pop-Up - {EF99BD32-C1FB-11D2-892F-0090271D4F88} - C:\Program Files\Yahoo!\Companion\Installs\cpn\yt.dll
O3 - Toolbar: avast! WebRep - {8E5E2654-AD2D-48bf-AC2D-D17F00898D06} - C:\Program Files\AVAST Software\Avast\aswWebRepIE.dll
O3 - Toolbar: StartSearchToolBar - {7AC3E13B-3BCA-4158-B330-F66DBB03C1B5} - C:\Program Files\StartSearch plugin\ssBarLcher.dll
O4 - HKLM\..\Run: [Windows Defender] %ProgramFiles%\Windows Defender\MSASCui.exe -hide
O4 - HKLM\..\Run: [SynTPStart] C:\Program Files\Synaptics\SynTP\SynTPStart.exe
O4 - HKLM\..\Run: [Adobe Reader Speed Launcher] "C:\Program Files\Adobe\Reader 8.0\Reader\Reader_sl.exe"
O4 - HKLM\..\Run: [eAudio] "C:\Acer\Empowering Technology\eAudio\eAudio.exe"
O4 - HKLM\..\Run: [NvSvc] RUNDLL32.EXE C:\Windows\system32\nvsvc.dll,nvsvcStart
O4 - HKLM\..\Run: [NvCplDaemon] RUNDLL32.EXE C:\Windows\system32\NvCpl.dll,NvStartup
O4 - HKLM\..\Run: [NvMediaCenter] RUNDLL32.EXE C:\Windows\system32\NvMcTray.dll,NvTaskbarInit
O4 - HKLM\..\Run: [RtHDVCpl] RtHDVCpl.exe
O4 - HKLM\..\Run: [Skytel] Skytel.exe
O4 - HKLM\..\Run: [IAAnotif] C:\Program Files\Intel\Intel Matrix Storage Manager\iaanotif.exe
O4 - HKLM\..\Run: [SetPanel] C:\Acer\APanel\APanel.cmd
O4 - HKLM\..\Run: [LManager] C:\PROGRA~1\LAUNCH~1\QtZgAcer.EXE
O4 - HKLM\..\Run: [PlayMovie] "C:\Program Files\Acer Arcade Deluxe\Play Movie\PMVService.exe"
O4 - HKLM\..\Run: [WarReg_PopUp] C:\Acer\WR_PopUp\WarReg_PopUp.exe
O4 - HKLM\..\Run: [PLFSet] rundll32.exe C:\Windows\PLFSet.dll,PLFDefSetting
O4 - HKLM\..\Run: [Acer Tour Reminder] C:\Acer\AcerTour\Reminder.exe
O4 - HKLM\..\Run: [AliceRV_McciTrayApp] C:\Program Files\Alice ti aiuta\McciTrayApp.exe
O4 - HKLM\..\Run: [QuickTime Task] "C:\Program Files\QuickTime\QTTask.exe" -atboottime
O4 - HKLM\..\Run: [Symantec PIF AlertEng] "C:\Program Files\Common Files\Symantec Shared\PIF\{B8E1DD85-8582-4c61-B58F-2F227FCA9A08}\PIFSvc.exe" /a /m "C:\Program Files\Common Files\Symantec Shared\PIF\{B8E1DD85-8582-4c61-B58F-2F227FCA9A08}\AlertEng.dll"
O4 - HKLM\..\Run: [avast] "C:\Program Files\AVAST Software\Avast\avastUI.exe" /nogui
O4 - HKLM\..\Run: [APSDaemon] "C:\Program Files\Common Files\Apple\Apple Application Support\APSDaemon.exe"
O4 - HKLM\..\Run: [iTunesHelper] "C:\Program Files\iTunes\iTunesHelper.exe"
O4 - HKLM\..\Run: [SunJavaUpdateSched] "C:\Program Files\Common Files\Java\Java Update\jusched.exe"
O4 - HKCU\..\Run: [Sidebar] C:\Program Files\Windows Sidebar\sidebar.exe /autoRun
O4 - HKCU\..\Run: [DAEMON Tools Lite] "C:\Program Files\DAEMON Tools Lite\DTLite.exe" -autorun
O4 - HKCU\..\Run: [EA Core] "C:\Program Files\Electronic Arts\EADM\Core.exe" -silent
O4 - HKUS\S-1-5-19\..\Run: [Sidebar] %ProgramFiles%\Windows Sidebar\Sidebar.exe /detectMem (User 'SERVIZIO LOCALE')
O4 - HKUS\S-1-5-19\..\Run: [WindowsWelcomeCenter] rundll32.exe oobefldr.dll,ShowWelcomeCenter (User 'SERVIZIO LOCALE')
O4 - HKUS\S-1-5-20\..\Run: [Sidebar] %ProgramFiles%\Windows Sidebar\Sidebar.exe /detectMem (User 'SERVIZIO DI RETE')
O4 - HKUS\S-1-5-18\..\Run: [Acer Tour Reminder] C:\Acer\AcerTour\Reminder.exe (User 'SYSTEM')
O4 - HKUS\.DEFAULT\..\Run: [Acer Tour Reminder] C:\Acer\AcerTour\Reminder.exe (User 'Default user')
O4 - Global Startup: Acer VCM.lnk = ?
O4 - Global Startup: Empowering Technology Launcher.lnk = ?
O8 - Extra context menu item: E&sporta in Microsoft Excel - res://C:\PROGRA~1\MICROS~2\Office12\EXCEL.EXE/3000
O9 - Extra button: Invia a OneNote - {2670000A-7350-4f3c-8081-5663EE0C6C49} - C:\PROGRA~1\MICROS~2\Office12\ONBttnIE.dll
O9 - Extra 'Tools' menuitem: I&nvia a OneNote - {2670000A-7350-4f3c-8081-5663EE0C6C49} - C:\PROGRA~1\MICROS~2\Office12\ONBttnIE.dll
O9 - Extra button: Research - {92780B25-18CC-41C8-B9BE-3C9C571A8263} - C:\PROGRA~1\MICROS~2\Office12\REFIEBAR.DLL
O22 - SharedTaskScheduler: Component Categories cache daemon - {8C7461EF-2B13-11d2-BE35-3078302C2030} - C:\Windows\system32\browseui.dll
O23 - Service: Apple Mobile Device - Apple Inc. - C:\Program Files\Common Files\Apple\Mobile Device Support\AppleMobileDeviceService.exe
O23 - Service: avast! Antivirus - AVAST Software - C:\Program Files\AVAST Software\Avast\AvastSvc.exe
O23 - Service: Servizio Bonjour (Bonjour Service) - Apple Inc. - C:\Program Files\Bonjour\mDNSResponder.exe
O23 - Service: Symantec Lic NetConnect service (CLTNetCnService) - Unknown owner - C:\Program Files\Common Files\Symantec Shared\ccSvcHst.exe (file missing)
O23 - Service: eRecovery Service (eRecoveryService) - Acer Inc. - C:\Acer\Empowering Technology\eRecovery\eRecoveryService.exe
O23 - Service: Servizio di Google Update (gupdate) (gupdate) - Google Inc. - C:\Program Files\Google\Update\GoogleUpdate.exe
O23 - Service: Servizio Google Update (gupdatem) (gupdatem) - Google Inc. - C:\Program Files\Google\Update\GoogleUpdate.exe
O23 - Service: Intel(R) Matrix Storage Event Monitor (IAANTMON) - Intel Corporation - C:\Program Files\Intel\Intel Matrix Storage Manager\IAANTMon.exe
O23 - Service: Servizio iPod (iPod Service) - Apple Inc. - C:\Program Files\iPod\bin\iPodService.exe
O23 - Service: LightScribeService Direct Disc Labeling Service (LightScribeService) - Hewlett-Packard Company - C:\Program Files\Common Files\LightScribe\LSSrvc.exe
O23 - Service: LiveUpdate - Symantec Corporation - C:\PROGRA~1\Symantec\LIVEUP~1\LUCOMS~1.EXE
O23 - Service: LiveUpdate Notice Service Ex (LiveUpdate Notice Ex) - Unknown owner - C:\Program Files\Common Files\Symantec Shared\ccSvcHst.exe (file missing)
O23 - Service: LiveUpdate Notice Service - Symantec Corporation - C:\Program Files\Common Files\Symantec Shared\PIF\{B8E1DD85-8582-4c61-B58F-2F227FCA9A08}\PIFSvc.exe
O23 - Service: MobilityService - Unknown owner - C:\Acer\Mobility Center\MobilityService.exe
O23 - Service: Cyberlink RichVideo Service(CRVS) (RichVideo) - Unknown owner - C:\Program Files\CyberLink\Shared Files\RichVideo.exe
O23 - Service: Raw Socket Service (RS_Service) - Acer Inc. - C:\Program Files\Acer\Acer VCM\RS_Service.exe
O23 - Service: Utilità di pianificazione di LiveUpdate automatico - Symantec Corporation - C:\Program Files\Symantec\LiveUpdate\ALUSchedulerSvc.exe
O23 - Service: XAudioService - Conexant Systems, Inc. - C:\Windows\system32\DRIVERS\xaudio.exe

--
End of file - 10300 bytes

C'è qualcosa da eliminare, poi hai un sacco di programmi in avvio che creano solo rallentamenti.
Installa QUESTO lo aggiorni e poi fai una scansione COMPLETA non veloce, elimina quello che trova, posta il log.
Chiudi tutti i programmi e disconnesso lanci HJT e clicca sul secondo pulsante: Do a system scan only poi metti la spunta alle voci che ti indico e alla fine clic su Fix checked.

R1 - HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings,ProxyOverride = *.local
O2 - BHO: IE5BarLauncherBHO Class - {78F3A323-798E-4AEA-9A57-88F4B05FD5DD} - C:\Program Files\StartSearch plugin\ssBarLcher.dll
O2 - BHO: (no name) - {83A2F9B1-01A2-4AA5-87D1-45B6B8505E96} - (no file)
O3 - Toolbar: avast! WebRep - {8E5E2654-AD2D-48bf-AC2D-D17F00898D06} - C:\Program Files\AVAST Software\Avast\aswWebRepIE.dll
O3 - Toolbar: StartSearchToolBar - {7AC3E13B-3BCA-4158-B330-F66DBB03C1B5} - C:\Program Files\StartSearch plugin\ssBarLcher.dll
O4 - HKLM\..\Run: [Adobe Reader Speed Launcher] "C:\Program Files\Adobe\Reader 8.0\Reader\Reader_sl.exe"
O4 - HKLM\..\Run: [SetPanel] C:\Acer\APanel\APanel.cmd
O4 - HKLM\..\Run: [LManager] C:\PROGRA~1\LAUNCH~1\QtZgAcer.EXE
O4 - HKLM\..\Run: [WarReg_PopUp] C:\Acer\WR_PopUp\WarReg_PopUp.exe
O4 - HKLM\..\Run: [PLFSet] rundll32.exe C:\Windows\PLFSet.dll,PLFDefSetting
O4 - HKLM\..\Run: [Acer Tour Reminder] C:\Acer\AcerTour\Reminder.exe
O4 - HKLM\..\Run: [AliceRV_McciTrayApp] C:\Program Files\Alice ti aiuta\McciTrayApp.exe
O4 - HKLM\..\Run: [QuickTime Task] "C:\Program Files\QuickTime\QTTask.exe" -atboottime
O4 - HKLM\..\Run: [APSDaemon] "C:\Program Files\Common Files\Apple\Apple Application Support\APSDaemon.exe"
O4 - HKLM\..\Run: [iTunesHelper] "C:\Program Files\iTunes\iTunesHelper.exe"
O4 - HKLM\..\Run: [SunJavaUpdateSched] "C:\Program Files\Common Files\Java\Java Update\jusched.exe"
O4 - HKCU\..\Run: [Sidebar] C:\Program Files\Windows Sidebar\sidebar.exe /autoRun
O4 - HKCU\..\Run: [DAEMON Tools Lite] "C:\Program Files\DAEMON Tools Lite\DTLite.exe" -autorun
O4 - HKCU\..\Run: [EA Core] "C:\Program Files\Electronic Arts\EADM\Core.exe" -silent
O4 - HKUS\S-1-5-19\..\Run: [Sidebar] %ProgramFiles%\Windows Sidebar\Sidebar.exe /detectMem (User 'SERVIZIO LOCALE')
O4 - HKUS\S-1-5-19\..\Run: [WindowsWelcomeCenter] rundll32.exe oobefldr.dll,ShowWelcomeCenter (User 'SERVIZIO LOCALE')
O4 - HKUS\S-1-5-20\..\Run: [Sidebar] %ProgramFiles%\Windows Sidebar\Sidebar.exe /detectMem (User 'SERVIZIO DI RETE')
O4 - HKUS\S-1-5-18\..\Run: [Acer Tour Reminder] C:\Acer\AcerTour\Reminder.exe (User 'SYSTEM')
O4 - HKUS\.DEFAULT\..\Run: [Acer Tour Reminder] C:\Acer\AcerTour\Reminder.exe (User 'Default user')
O4 - Global Startup: Acer VCM.lnk = ?
O4 - Global Startup: Empowering Technology Launcher.lnk = ?

Poi fai una pulizia con Ccleaner compreso il registro.
Lo spegnimento del pc può dipendere anche da surriscaldamento, controlla le temperature.
Fai anche uno scandisk approfondito.

Mbam ha già eliminato qualcosa, ora esegui quello che ti ho suggerito prima.