ho il pc che s'impalla rimane fermo x qualche secondo e poi riparte mi date un occhiata al Logfile of HijackThis v1.99.1
Scan saved at 13.05.56, on 13/04/2007
Platform: Windows XP SP2 (WinNT 5.01.2600)
MSIE: Internet Explorer v6.00 SP2 (6.00.2900.2180)

Running processes:
C:\WINDOWS\System32\smss.exe
C:\WINDOWS\system32\winlogon.exe
C:\WINDOWS\system32\services.exe
C:\WINDOWS\system32\lsass.exe
C:\WINDOWS\system32\Ati2evxx.exe
C:\WINDOWS\system32\svchost.exe
C:\WINDOWS\System32\svchost.exe
C:\WINDOWS\system32\svchost.exe
C:\WINDOWS\system32\spoolsv.exe
C:\Programmi\Alwil Software\Avast4\aswUpdSv.exe
C:\Programmi\Alwil Software\Avast4\ashServ.exe
C:\WINDOWS\system32\oodag.exe
C:\Programmi\SiteAdvisor\6066\SAService.exe
C:\WINDOWS\system32\Ati2evxx.exe
C:\WINDOWS\Explorer.EXE
C:\WINDOWS\system32\svchost.exe
C:\WINDOWS\system32\rundll32.exe
C:\PROGRA~1\ALWILS~1\Avast4\ashDisp.exe
C:\Programmi\SiteAdvisor\6066\SiteAdv.exe
C:\WINDOWS\system32\ctfmon.exe
C:\Programmi\Alwil Software\Avast4\ashMaiSv.exe
C:\Programmi\Alwil Software\Avast4\ashWebSv.exe
C:\Programmi\eMule\emule.exe
C:\Programmi\Internet Explorer\iexplore.exe
C:\Programmi\Hijackthis\HijackThis.exe

R0 - HKCU\Software\Microsoft\Internet Explorer\Main,Start Page = http://www.libero.it/
R0 - HKCU\Software\Microsoft\Internet Explorer\Main,Local Page =
R0 - HKLM\Software\Microsoft\Internet Explorer\Main,Local Page =
R0 - HKCU\Software\Microsoft\Internet Explorer\Toolbar,LinksFolderName = Collegamenti
O2 - BHO: Yahoo! Toolbar Helper - {02478D38-C3F9-4EFB-9B51-7695ECA05670} - C:\Programmi\Yahoo!\Companion\Installs\cpn\yt.dll
O2 - BHO: Adobe PDF Reader Link Helper - {06849E9F-C8D7-4D59-B87D-784B7D6BE0B3} - C:\Programmi\Adobe\Acrobat 7.0\ActiveX\AcroIEHelper.dll
O2 - BHO: (no name) - {089FD14D-132B-48FC-8861-0048AE113215} - C:\Programmi\SiteAdvisor\6066\SiteAdv.dll
O2 - BHO: ilsoftware Toolbar - {195c5499-fc60-40e3-8eec-72831ad6e5c5} - C:\Programmi\ilsoftware\tbils1.dll
O2 - BHO: (no name) - {53707962-6F74-2D53-2644-206D7942484F} - C:\PROGRA~1\SPYBOT~1\SDHelper.dll
O2 - BHO: SSVHelper Class - {761497BB-D6F0-462C-B6EB-D4DAF1D92D43} - C:\Programmi\Java\jre1.5.0_11\bin\ssv.dll
O2 - BHO: Windows Live Sign-in Helper - {9030D464-4C02-4ABF-8ECC-5164760863C6} - C:\Programmi\File comuni\Microsoft Shared\Windows Live\WindowsLiveLogin.dll
O3 - Toolbar: Yahoo! Toolbar - {EF99BD32-C1FB-11D2-892F-0090271D4F88} - C:\Programmi\Yahoo!\Companion\Installs\cpn\yt.dll
O3 - Toolbar: ilsoftware Toolbar - {195c5499-fc60-40e3-8eec-72831ad6e5c5} - C:\Programmi\ilsoftware\tbils1.dll
O3 - Toolbar: McAfee SiteAdvisor - {0BF43445-2F28-4351-9252-17FE6E806AA0} - C:\Programmi\SiteAdvisor\6066\SiteAdv.dll
O4 - HKLM\..\Run: [CnxTrApp] rundll32.exe "C:\Programmi\StarModem\StarModem USB Network\CnxTrApp.dll",AppEntry -REG "Conexant\Conexant USB Network"
O4 - HKLM\..\Run: [avast!] C:\PROGRA~1\ALWILS~1\Avast4\ashDisp.exe
O4 - HKLM\..\Run: [SiteAdvisor] C:\Programmi\SiteAdvisor\6066\SiteAdv.exe
O4 - HKLM\..\Run: [MSConfig] C:\WINDOWS\PCHealth\HelpCtr\Binaries\MSConfig.exe /auto
O4 - HKCU\..\Run: [CTFMON.EXE] C:\WINDOWS\system32\ctfmon.exe
O4 - Startup: ERUNT AutoBackup.lnk = C:\Programmi\ERUNT\AUTOBACK.EXE
O8 - Extra context menu item: E&sporta in Microsoft Excel - res://C:\PROGRA~1\MICROS~2\OFFICE11\EXCEL.EXE/3000
O9 - Extra button: (no name) - {08B0E5C0-4FCB-11CF-AAA5-00401C608501} - C:\Programmi\Java\jre1.5.0_11\bin\ssv.dll
O9 - Extra 'Tools' menuitem: Sun Java Console - {08B0E5C0-4FCB-11CF-AAA5-00401C608501} - C:\Programmi\Java\jre1.5.0_11\bin\ssv.dll
O9 - Extra button: Ricerche - {92780B25-18CC-41C8-B9BE-3C9C571A8263} - C:\PROGRA~1\MICROS~2\OFFICE11\REFIEBAR.DLL
O9 - Extra button: Messenger - {FB5F1910-F110-11d2-BB9E-00C04F795683} - C:\Programmi\Messenger\msmsgs.exe
O9 - Extra 'Tools' menuitem: Windows Messenger - {FB5F1910-F110-11d2-BB9E-00C04F795683} - C:\Programmi\Messenger\msmsgs.exe
O16 - DPF: {17492023-C23A-453E-A040-C7C580BBF700} - http://go.microsoft.com/fwlink/?linkid=39204
O16 - DPF: {193C772A-87BE-4B19-A7BB-445B226FE9A1} (ewidoOnlineScan Control) - http://downloads.ewido.net/ewidoOnlineScan.cab
O16 - DPF: {2BC66F54-93A8-11D3-BEB6-00105AA9B6AE} (Symantec AntiVirus scanner) - http://security.symantec.com/sscv6/SharedContent/vc/bin/AvSniff.cab
O16 - DPF: {4F1E5B1A-2A80-42CA-8532-2D05CB959537} (MSN Photo Upload Tool) - http://mukka1989.spaces.live.com//PhotoUpload/MsnPUpld.cab
O16 - DPF: {5D6F45B3-9043-443D-A792-115447494D24} (UnoCtrl Class) - http://messenger.zone.msn.com/IT-IT/a-UNO1/GAME_UNO1.cab
O16 - DPF: {6414512B-B978-451D-A0D8-FCFDF33E833C} (WUWebControl Class) - http://update.microsoft.com/windowsupdate/v6/V5Controls/en/x86/client/wuweb_site.cab?1165010330765
O16 - DPF: {644E432F-49D3-41A1-8DD5-E099162EEEC5} (Symantec RuFSI Utility Class) - http://security.symantec.com/sscv6/SharedContent/common/bin/cabsa.cab
O16 - DPF: {8436FE12-31DB-48BF-83BF-FE682F9160B4} (NanoInstaller Class) - http://www.nanoscan.com/cabs/nanoinst.cab
O16 - DPF: {8E0D4DE5-3180-4024-A327-4DFAD1796A8D} (MessengerStatsClient Class) - http://messenger.zone.msn.com/binary/MessengerStatsClient.cab31267.cab
O18 - Protocol: livecall - {828030A1-22C1-4009-854F-8E305202313F} - C:\PROGRA~1\MSNMES~1\MSGRAP~1.DLL
O18 - Protocol: msnim - {828030A1-22C1-4009-854F-8E305202313F} - C:\PROGRA~1\MSNMES~1\MSGRAP~1.DLL
O18 - Protocol: siteadvisor - {3A5DC592-7723-4EAA-9EE6-AF4222BCF879} - C:\Programmi\SiteAdvisor\6066\SiteAdv.dll
O20 - Winlogon Notify: WgaLogon - C:\WINDOWSO23 - Service: avast! iAVS4 Control Service (aswUpdSv) - Unknown owner - C:\Programmi\Alwil Software\Avast4\aswUpdSv.exe
O23 - Service: Ati HotKey Poller - ATI Technologies Inc. - C:\WINDOWS\system32\Ati2evxx.exe
O23 - Service: ATI Smart - Unknown owner - C:\WINDOWS\system32\ati2sgag.exe
O23 - Service: avast! Antivirus - Unknown owner - C:\Programmi\Alwil Software\Avast4\ashServ.exe
O23 - Service: avast! Mail Scanner - Unknown owner - C:\Programmi\Alwil Software\Avast4\ashMaiSv.exe" /service (file missing)
O23 - Service: avast! Web Scanner - Unknown owner - C:\Programmi\Alwil Software\Avast4\ashWebSv.exe" /service (file missing)
O23 - Service: Windows Presentation Foundation Font Cache 3.0.0.0 (FontCache3.0.0.0) - Unknown owner - C:\WINDOWS\Microsoft.Net\Framework\v3.0\WPF\PresentationFontCache.exe (file missing)
O23 - Service: Google Updater Service (gusvc) - Google - C:\Programmi\Google\Common\Google Updater\GoogleUpdaterService.exe
O23 - Service: Windows CardSpace (idsvc) - Unknown owner - C:\WINDOWS\Microsoft.NET\Framework\v3.0\Windows Communication Foundation\infocard.exe (file missing)
O23 - Service: O&O Defrag - O&O Software GmbH - C:\WINDOWS\system32\oodag.exe
O23 - Service: SiteAdvisor Service - McAfee, Inc. - C:\Programmi\SiteAdvisor\6066\SAService.exe
O23 - Service: TrueVector Internet Monitor (vsmon) - Unknown owner - c:\documents and settings\io.io-e05fd2476a1d\desktop\programmi\zonealarm\repair\vsmon.exe (file missing)
grazie.

Sono sicuramente da "fixare" le voci seguenti:

O23 - Service: avast! Mail Scanner - Unknown owner - C:\Programmi\Alwil Software\Avast4\ashMaiSv.exe" /service (file missing)
O23 - Service: avast! Web Scanner - Unknown owner - C:\Programmi\Alwil Software\Avast4\ashWebSv.exe" /service (file missing)
O23 - Service: Windows Presentation Foundation Font Cache 3.0.0.0 (FontCache3.0.0.0) - Unknown owner - C:\WINDOWS\Microsoft.Net\Framework\v3.0\WPF\PresentationFontCache.exe (file missing)
O23 - Service: Windows CardSpace (idsvc) - Unknown owner - C:\WINDOWS\Microsoft.NET\Framework\v3.0\Windows Communication Foundation\infocard.exe (file missing)
O23 - Service: TrueVector Internet Monitor (vsmon) - Unknown owner - c:\documents and settings\io.io-e05fd2476a1d\desktop\programmi\zonealarm\repair\vsmon.exe (file missing)

In ogni caso, aspetta di sentire altre opinioni prima di stabilire che le altre vanno davvero bene.
Non hai un valido firewall installato sul Sistema: installane uno di corsa o andrai sempre e comunque incontro a seri guai.