Ciao,
ho aggiornato il sistema e fatto una scansione con BitDefender che non riporta infezioni. Riporto il log di VirIt:

11/09/2006 - 12:41:48

[SCANSIONE DEL REGISTRO]
{2a6af021-17a2-4014-8624-cf6015f82fad} Infetto da BHO.Agent.BA

[C:]
MASTER BOOT RECORD: OK
BOOT SECTOR: OK


Chiavi Registro infette: 1.
Files Infetti: 0.
Files Sospetti: 0.
Files Analizzati: 64445.
Files Totali: 64445.
Chiavi Registro rimosse: 0.
Virus Rimossi: 0.

purtroppo non riesco a rimuovere l'infezione causa scadenza del periodo di prova.

Aggiungo nuovamente anche il log di Hijack This:

Logfile of HijackThis v1.99.1
Scan saved at 13.32.31, on 11/09/2006
Platform: Windows XP SP1 (WinNT 5.01.2600)
MSIE: Internet Explorer v6.00 SP1 (6.00.2800.1106)

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\spoolsv.exe
C:\Programmi\Alwil Software\Avast4\aswUpdSv.exe
C:\Programmi\Alwil Software\Avast4\ashServ.exe
C:\WINDOWS\System32\DRIVERS\CDANTSRV.EXE
C:\Programmi\ewido anti-malware\ewidoctrl.exe
C:\Programmi\Alcohol Soft\Alcohol 120\StarWind\StarWindService.exe
C:\VEXPLITE\viritsvc.exe
C:\WINDOWS\system32\ZoneLabs\vsmon.exe
C:\WINDOWS\SYSTEM32\Ati2evxx.exe
C:\WINDOWS\Explorer.EXE
C:\Programmi\Alwil Software\Avast4\ashWebSv.exe
C:\Programmi\Alwil Software\Avast4\ashMaiSv.exe
C:\WINDOWS\SOUNDMAN.EXE
C:\WINDOWS\System32\WgaTray.exe
C:\Programmi\Adaptec\Easy CD Creator 5\DirectCD\DirectCD.exe
C:\Programmi\File comuni\Real\Update_OB\realsched.exe
C:\Programmi\Hewlett-Packard\Digital Imaging\bin\hpotdd01.exe
C:\Programmi\DAEMON Tools\daemon.exe
C:\PROGRA~1\ALWILS~1\Avast4\ashDisp.exe
C:\Programmi\Hewlett-Packard\HP Software Update\HPWuSchd.exe
C:\Programmi\HP\hpcoretech\hpcmpmgr.exe
C:\WINDOWS\System32\spool\drivers\w32x86\3\hpztsb09.exe
C:\VEXPLITE\MONLITE.EXE
C:\Programmi\Spyware Terminator\SpywareTerminatorShield.exe
C:\Programmi\Zone Labs\ZoneAlarm\zlclient.exe
C:\Programmi\Messenger\msmsgs.exe
C:\Programmi\Le Robert\Le Petit Robert\prhyper.exe
C:\Programmi\SUPERAntiSpyware\SUPERAntiSpyware.exe
C:\Programmi\WinClamAVShield\sp_clam.exe
C:\Programmi\Alice ti aiuta\bin\mpbtn.exe
C:\VEXPLITE\VIRITEXP.EXE
C:\WINDOWS\System32\wuauclt.exe
C:\Programmi\Alwil Software\Avast4\setup\avast.setup
C:\Programmi\Internet Explorer\iexplore.exe
C:\Documents and Settings\Administrator\Desktop\psx - pc\antivirus\HijackThis.exe

R0 - HKCU\Software\Microsoft\Internet Explorer\Main,Start Page = http://www.ebay.it/
O2 - BHO: AcroIEHlprObj Class - {06849E9F-C8D7-4D59-B87D-784B7D6BE0B3} - C:\Programmi\Adobe\Acrobat 5.0\Reader\ActiveX\AcroIEHelper.ocx
O2 - BHO: Google Toolbar Helper - {AA58ED58-01DD-4d91-8333-CF10577473F7} - c:\programmi\google\googletoolbar2.dll
O3 - Toolbar: &Google - {2318C2B1-4965-11d4-9B18-009027A5CD4F} - c:\programmi\google\googletoolbar2.dll
O3 - Toolbar: &Radio - {8E718888-423F-11D2-876E-00A0C9082467} - C:\WINDOWS\System32\msdxm.ocx
O4 - HKLM\..\Run: [SoundMan] SOUNDMAN.EXE
O4 - HKLM\..\Run: [NeroCheck] C:\WINDOWS\system32\NeroCheck.exe
O4 - HKLM\..\Run: [QuickTime Task] "C:\Programmi\QuickTime\qttask.exe" -atboottime
O4 - HKLM\..\Run: [AdaptecDirectCD] "C:\Programmi\Adaptec\Easy CD Creator 5\DirectCD\DirectCD.exe"
O4 - HKLM\..\Run: [SSC_UserPrompt] C:\Programmi\File comuni\Symantec Shared\Security Center\UsrPrmpt.exe
O4 - HKLM\..\Run: [TkBellExe] "C:\Programmi\File comuni\Real\Update_OB\realsched.exe" -osboot
O4 - HKLM\..\Run: [DeviceDiscovery] C:\Programmi\Hewlett-Packard\Digital Imaging\bin\hpotdd01.exe
O4 - HKLM\..\Run: [DAEMON Tools] "C:\Programmi\DAEMON Tools\daemon.exe" -lang 1033
O4 - HKLM\..\Run: [avast!] C:\PROGRA~1\ALWILS~1\Avast4\ashDisp.exe
O4 - HKLM\..\Run: [HP Software Update] "C:\Programmi\Hewlett-Packard\HP Software Update\HPWuSchd.exe"
O4 - HKLM\..\Run: [HP Component Manager] "C:\Programmi\HP\hpcoretech\hpcmpmgr.exe"
O4 - HKLM\..\Run: [HPDJ Taskbar Utility] C:\WINDOWS\System32\spool\drivers\w32x86\3\hpztsb09.exe
O4 - HKLM\..\Run: [VIRIT LITE MONITOR] C:\VEXPLITE\MONLITE.EXE
O4 - HKLM\..\Run: [SpywareTerminator] "C:\Programmi\Spyware Terminator\SpywareTerminatorShield.exe"
O4 - HKLM\..\Run: [Zone Labs Client] C:\Programmi\Zone Labs\ZoneAlarm\zlclient.exe
O4 - HKCU\..\Run: [MSMSGS] "C:\Programmi\Messenger\msmsgs.exe" /background
O4 - HKCU\..\Run: [Le Petit Robert Hyperappel] C:\Programmi\Le Robert\Le Petit Robert\prhyper.exe
O4 - HKCU\..\Run: [SUPERAntiSpyware] C:\Programmi\SUPERAntiSpyware\SUPERAntiSpyware.exe
O4 - Global Startup: Alice ti aiuta.lnk = C:\Programmi\Alice ti aiuta\bin\matcli.exe
O4 - Global Startup: Microsoft Office.lnk = C:\Programmi\Microsoft Office\Office\OSA9.EXE
O8 - Extra context menu item: &Cerca con Google - res://c:\programmi\google\GoogleToolbar2.dll/cmsearch.html
O8 - Extra context menu item: &Traduci parola in italiano - res://c:\programmi\google\GoogleToolbar2.dll/cmwordtrans.html
O8 - Extra context menu item: Link a ritroso - res://c:\programmi\google\GoogleToolbar2.dll/cmbacklinks.html
O8 - Extra context menu item: Pagine simili - res://c:\programmi\google\GoogleToolbar2.dll/cmsimilar.html
O8 - Extra context menu item: Versione cache della pagina - res://c:\programmi\google\GoogleToolbar2.dll/cmcache.html
O9 - Extra button: Messenger - {FB5F1910-F110-11d2-BB9E-00C04F795683} - C:\Programmi\Messenger\MSMSGS.EXE
O9 - Extra 'Tools' menuitem: Messenger - {FB5F1910-F110-11d2-BB9E-00C04F795683} - C:\Programmi\Messenger\MSMSGS.EXE
O16 - DPF: {5D86DDB5-BDF9-441B-9E9E-D4730F4EE499} (BDSCANONLINE Control) - http://download.bitdefender.com/resources/scan8/oscan8.cab
O16 - DPF: {6E32070A-766D-4EE6-879C-DC1FA91D2FC3} (MUWebControl Class) - http://update.microsoft.com/microsoftupdate/v6/V5Controls/en/x86/client/muweb_site.cab?1157020176234
O17 - HKLM\System\CCS\Services\Tcpip\..\{CF261574-1FDB-4EC5-BC58-28017B42AC11}: NameServer = 85.37.17.5 85.38.28.77
O20 - Winlogon Notify: SASWinLogon - C:\Programmi\SUPERAntiSpyware\SASWINLO.dll
O20 - Winlogon Notify: WgaLogon - C:\WINDOWS\SYSTEM32\WgaLogon.dll
O23 - Service: avast! iAVS4 Control Service (aswUpdSv) - Unknown owner - C:\Programmi\Alwil Software\Avast4\aswUpdSv.exe
O23 - Service: Ati HotKey Poller - Unknown owner - 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: C-DillaSrv - C-Dilla Ltd - C:\WINDOWS\System32\DRIVERS\CDANTSRV.EXE
O23 - Service: ewido security suite control - ewido networks - C:\Programmi\ewido anti-malware\ewidoctrl.exe
O23 - Service: IMAPI CD-Burning COM Service (ImapiService) - Roxio Inc. - C:\WINDOWS\System32\ImapiRox.exe
O23 - Service: StarWind iSCSI Service (StarWindService) - Rocket Division Software - C:\Programmi\Alcohol Soft\Alcohol 120\StarWind\StarWindService.exe
O23 - Service: SymWMI Service (SymWSC) - Symantec Corporation - C:\Programmi\File comuni\Symantec Shared\Security Center\SymWSC.exe
O23 - Service: Virit eXplorer Lite (viritsvclite) - TG Soft Sas www.tgsoft.it - C:\VEXPLITE\viritsvc.exe
O23 - Service: TrueVector Internet Monitor (vsmon) - Zone Labs, LLC - C:\WINDOWS\system32\ZoneLabs\vsmon.exe

Un'ultima cosa: ho alcune applicazioni random che mi fanno pensare a Link Optimizer. I nomi dei file sono tipo: nixc.exe, bnfxke.exe, lstmhtvf.exe....come posso fare per controllare se si tratta effettivamente di un virus?Grazie grazie!
Ciao!