Chiedo scusa se non ho letto fin da subito il messaggio in rilievo "guida alla rimozione malware". Comunque ora ho eseguito la scansione con Malwarebytes' Anti-Malware (nessun file infetto trovato), poi con Kaspersky Scanner (idem). Questo è il log di HijackThis:

Logfile of Trend Micro HijackThis v2.0.2
Scan saved at 13.09.51, on 12/11/2011
Platform: Windows XP SP3 (WinNT 5.01.2600)
MSIE: Internet Explorer v8.00 (8.00.6001.18702)
Boot mode: Normal

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\svchost.exe
C:\WINDOWS\System32\svchost.exe
C:\Programmi\File comuni\Symantec Shared\ccSetMgr.exe
C:\Programmi\File comuni\Symantec Shared\ccEvtMgr.exe
C:\WINDOWS\system32\spoolsv.exe
C:\WINDOWS\Explorer.EXE
C:\WINDOWS\SOUNDMAN.EXE
C:\WINDOWS\system32\keyhook.exe
C:\Programmi\Java\jre6\bin\jusched.exe
C:\Programmi\File comuni\Adobe\ARM\1.0\AdobeARM.exe
C:\Programmi\HP\hpcoretech\hpcmpmgr.exe
C:\Programmi\HP\HP Software Update\HPWuSchd2.exe
C:\Programmi\Winamp\winampa.exe
C:\Programmi\File comuni\Symantec Shared\ccApp.exe
C:\Programmi\Samsung\Samsung Media Studio 5\SMSTray.exe
C:\Programmi\MarkAny\ContentSafer\MAAgent.exe
C:\Programmi\Messenger\msmsgs.exe
C:\WINDOWS\system32\ctfmon.exe
C:\Programmi\HP\Digital Imaging\bin\hpqtra08.exe
C:\WINDOWS\system32\sistray.exe
C:\Programmi\OpenOffice.org 3\program\soffice.exe
C:\Programmi\OpenOffice.org 3\program\soffice.bin
C:\Programmi\Java\jre6\bin\jqs.exe
C:\Programmi\File comuni\LightScribe\LSSrvc.exe
C:\Programmi\Norton AntiVirus\navapsvc.exe
C:\Programmi\Norton AntiVirus\SAVScan.exe
C:\WINDOWS\System32\svchost.exe
C:\Programmi\HP\Digital Imaging\bin\hpqSTE08.exe
C:\WINDOWS\system32\wscntfy.exe
C:\Programmi\HP\Digital Imaging\Product Assistant\bin\hprblog.exe
C:\Programmi\internet explorer\iexplore.exe
C:\WINDOWS\System32\wbem\wmiapsrv.exe
C:\WINDOWS\system32\wuauclt.exe
C:\Programmi\Java\jre6\bin\jucheck.exe
C:\Programmi\internet explorer\iexplore.exe
C:\Programmi\Internet Explorer\IEXPLORE.EXE
C:\Programmi\internet explorer\iexplore.exe
C:\Programmi\Trend Micro\HijackThis\HijackThis.exe
C:\Programmi\Internet Explorer\iexplore.exe

R0 - HKCU\Software\Microsoft\Internet Explorer\Main,Start Page = http://www.libero.it/
R1 - HKLM\Software\Microsoft\Internet Explorer\Main,Default_Page_URL = http://go.microsoft.com/fwlink/?LinkId=69157
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://go.microsoft.com/fwlink/?LinkId=69157
R0 - HKCU\Software\Microsoft\Internet Explorer\Toolbar,LinksFolderName = Collegamenti
O2 - BHO: AcroIEHelperStub - {18DF081C-E8AD-4283-A596-FA578C2EBDC3} - C:\Programmi\File comuni\Adobe\Acrobat\ActiveX\AcroIEHelperShim.dll
O2 - BHO: NAV Helper - {BDF3E430-B101-42AD-A544-FADC6B084872} - C:\Programmi\Norton AntiVirus\NavShExt.dll
O2 - BHO: Java(tm) Plug-In 2 SSV Helper - {DBC80044-A445-435b-BC74-9C25C1C588A9} - C:\Programmi\Java\jre6\bin\jp2ssv.dll
O2 - BHO: JQSIEStartDetectorImpl - {E7E6F031-17CE-4C07-BC86-EABFE594F69C} - C:\Programmi\Java\jre6\lib\deploy\jqs\ie\jqs_plugi n.dll
O3 - Toolbar: Norton AntiVirus - {42CDD1BF-3FFB-4238-8AD1-7859DF00B1D6} - C:\Programmi\Norton AntiVirus\NavShExt.dll
O4 - HKLM\..\Run: [SoundMan] SOUNDMAN.EXE
O4 - HKLM\..\Run: [SiSPower] Rundll32.exe SiSPower.dll,ModeAgent
O4 - HKLM\..\Run: [SiS Windows KeyHook] C:\WINDOWS\system32\keyhook.exe
O4 - HKLM\..\Run: [SunJavaUpdateSched] "C:\Programmi\Java\jre6\bin\jusched.exe"
O4 - HKLM\..\Run: [Adobe Reader Speed Launcher] "C:\Programmi\Adobe\Reader 9.0\Reader\Reader_sl.exe"
O4 - HKLM\..\Run: [Adobe ARM] "C:\Programmi\File comuni\Adobe\ARM\1.0\AdobeARM.exe"
O4 - HKLM\..\Run: [HP Component Manager] "C:\Programmi\HP\hpcoretech\hpcmpmgr.exe"
O4 - HKLM\..\Run: [HP Software Update] C:\Programmi\HP\HP Software Update\HPWuSchd2.exe
O4 - HKLM\..\Run: [WinampAgent] C:\Programmi\Winamp\winampa.exe
O4 - HKLM\..\Run: [ccApp] "C:\Programmi\File comuni\Symantec Shared\ccApp.exe"
O4 - HKLM\..\Run: [Symantec NetDriver Monitor] C:\PROGRA~1\SYMNET~1\SNDMon.exe
O4 - HKLM\..\Run: [SMSTray] C:\Programmi\Samsung\Samsung Media Studio 5\SMSTray.exe
O4 - HKLM\..\Run: [MAAgent] C:\Programmi\MarkAny\ContentSafer\MAAgent.exe
O4 - HKLM\..\Run: [NeroFilterCheck] C:\WINDOWS\system32\NeroCheck.exe
O4 - HKCU\..\Run: [MSMSGS] "C:\Programmi\Messenger\msmsgs.exe" /background
O4 - HKCU\..\Run: [ctfmon.exe] C:\WINDOWS\system32\ctfmon.exe
O4 - HKUS\S-1-5-18\..\Run: [CTFMON.EXE] C:\WINDOWS\System32\CTFMON.EXE (User 'SYSTEM')
O4 - HKUS\S-1-5-18\..\Run: [ALUAlert] C:\Programmi\Symantec\LiveUpdate\ALUNotify.exe (User 'SYSTEM')
O4 - HKUS\.DEFAULT\..\Run: [CTFMON.EXE] C:\WINDOWS\System32\CTFMON.EXE (User 'Default user')
O4 - Startup: OpenOffice.org 3.1.lnk = C:\Programmi\OpenOffice.org 3\program\quickstart.exe
O4 - Global Startup: HP Digital Imaging Monitor.lnk = C:\Programmi\HP\Digital Imaging\bin\hpqtra08.exe
O4 - Global Startup: Utility Tray.lnk = C:\WINDOWS\system32\sistray.exe
O9 - Extra button: (no name) - {e2e2dd38-d088-4134-82b7-f2ba38496583} - C:\WINDOWS\Network Diagnostic\xpnetdiag.exe
O9 - Extra 'Tools' menuitem: @xpsp3res.dll,-20001 - {e2e2dd38-d088-4134-82b7-f2ba38496583} - C:\WINDOWS\Network Diagnostic\xpnetdiag.exe
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
O23 - Service: Symantec Event Manager (ccEvtMgr) - Symantec Corporation - C:\Programmi\File comuni\Symantec Shared\ccEvtMgr.exe
O23 - Service: Symantec Password Validation (ccPwdSvc) - Symantec Corporation - C:\Programmi\File comuni\Symantec Shared\ccPwdSvc.exe
O23 - Service: Symantec Settings Manager (ccSetMgr) - Symantec Corporation - C:\Programmi\File comuni\Symantec Shared\ccSetMgr.exe
O23 - Service: Java Quick Starter (JavaQuickStarterService) - Sun Microsystems, Inc. - C:\Programmi\Java\jre6\bin\jqs.exe
O23 - Service: LightScribeService Direct Disc Labeling Service (LightScribeService) - Hewlett-Packard Company - C:\Programmi\File comuni\LightScribe\LSSrvc.exe
O23 - Service: Servizio Norton AntiVirus Auto-Protect (navapsvc) - Symantec Corporation - C:\Programmi\Norton AntiVirus\navapsvc.exe
O23 - Service: Pml Driver HPZ12 - HP - C:\WINDOWS\system32\HPZipm12.exe
O23 - Service: SAVScan - Symantec Corporation - C:\Programmi\Norton AntiVirus\SAVScan.exe
O23 - Service: ScriptBlocking Service (SBService) - Symantec Corporation - C:\PROGRA~1\FILECO~1\SYMANT~1\SCRIPT~1\SBServ.exe
O23 - Service: Symantec Network Drivers Service (SNDSrvc) - Symantec Corporation - C:\Programmi\File comuni\Symantec Shared\SNDSrvc.exe

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End of file - 6792 bytes