Ho cercato di fare da solo seguendo i vari post, ma ho capito che esiste una soluzione diversa per ogni macchina.
Allego il log file dell'HJT aggiornato sperando nella vostra gentilezza.

Logfile of HijackThis v1.97.7
Scan saved at 11.47.51, on 05/05/2004
Platform: Windows XP (WinNT 5.01.2600)
MSIE: Internet Explorer v6.00 (6.00.2600.0000)

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\System32\Ati2evxx.exe
C:\Programmi\Norton AntiVirus\navapsvc.exe
C:\WINDOWS\system32\slserv.exe
C:\Programmi\Norton AntiVirus\SAVScan.exe
C:\WINDOWS\Explorer.EXE
C:\WINDOWS\System32\atiptaxx.exe
C:\Programmi\Apoint2K\Apoint.exe
C:\PROGRA~1\EzButton\CPATR10.EXE
C:\Programmi\File comuni\Symantec Shared\ccApp.exe
C:\WINDOWS\System32\ctfmon.exe
C:\Programmi\Messenger\msmsgs.exe
C:\Programmi\Microsoft Office\Office\1040\OLFSNT40.EXE
C:\Programmi\Apoint2K\Apntex.exe
C:\WINDOWS\System32\spool\DRIVERS\W32X86\3\E_SICN0 3.EXE
C:\WINDOWS\System32\wuauclt.exe
C:\WINDOWS\System32\svchost.exe
C:\Documents and Settings\Acer\Desktop\HijackThis.exe

R1 - HKCU\Software\Microsoft\Internet Explorer\Main,Search Bar = http://www.arianna.it/perie/hometestie.html
R1 - HKLM\Software\Microsoft\Internet Explorer\Main,Default_Page_URL = http://www.iol.it
R1 - HKCU\Software\Microsoft\Internet Explorer\Main,Window Title = Microsoft Internet Explorer fornito da Infostrada LIBERO
R1 - HKCU\Software\Microsoft\Windows\CurrentVersion\Int ernet Settings,ProxyServer = proxy.libero.it:8080
R1 - HKCU\Software\Microsoft\Windows\CurrentVersion\Int ernet Settings,ProxyOverride = abbonati.libero.it;www.libero.it;*.libero.;*.;;<local>
R0 - HKCU\Software\Microsoft\Internet Explorer\Toolbar,LinksFolderName = Collegamenti
O2 - BHO: (no name) - {06849E9F-C8D7-4D59-B87D-784B7D6BE0B3} - C:\Programmi\Adobe\Acrobat 5.0\Reader\ActiveX\AcroIEHelper.ocx
O2 - BHO: NAV Helper - {BDF3E430-B101-42AD-A544-FADC6B084872} - C:\Programmi\Norton AntiVirus\NavShExt.dll
O3 - Toolbar: &Radio - {8E718888-423F-11D2-876E-00A0C9082467} - C:\WINDOWS\System32\msdxm.ocx
O3 - Toolbar: Norton AntiVirus - {42CDD1BF-3FFB-4238-8AD1-7859DF00B1D6} - C:\Programmi\Norton AntiVirus\NavShExt.dll
O4 - HKLM\..\Run: [ATIModeChange] Ati2mdxx.exe
O4 - HKLM\..\Run: [AtiPTA] atiptaxx.exe
O4 - HKLM\..\Run: [Apoint] C:\Programmi\Apoint2K\Apoint.exe
O4 - HKLM\..\Run: [CPATR10] C:\PROGRA~1\EzButton\CPATR10.EXE
O4 - HKLM\..\Run: [ccApp] "C:\Programmi\File comuni\Symantec Shared\ccApp.exe"
O4 - HKCU\..\Run: [CTFMON.EXE] C:\WINDOWS\System32\ctfmon.exe
O4 - HKCU\..\Run: [MSMSGS] "C:\Programmi\Messenger\msmsgs.exe" /background
O4 - Global Startup: Microsoft Office.lnk = C:\Programmi\Microsoft Office\Office\OSA9.EXE
O4 - Global Startup: Adobe Gamma Loader.exe.lnk = C:\Programmi\File comuni\Adobe\Calibration\Adobe Gamma Loader.exe
O4 - Global Startup: Porta Symantec Fax Starter Edition.lnk = C:\Programmi\Microsoft Office\Office\1040\OLFSNT40.EXE
O4 - Global Startup: EPSON Status Monitor 3 Environment Check.lnk = C:\WINDOWS\system32\spool\drivers\w32x86\3\E_SRCV0 3.EXE
O12 - Plugin for .mov: C:\Programmi\Internet Explorer\PLUGINS\npqtplugin.dll
O12 - Plugin for .mp3: C:\Programmi\Internet Explorer\PLUGINS\npqtplugin3.dll
O12 - Plugin for .mpeg: C:\Programmi\Internet Explorer\PLUGINS\npqtplugin3.dll
O14 - IERESET.INF: START_PAGE_URL=http://www.iol.it
O17 - HKLM\System\CCS\Services\Tcpip\..\{869689FF-8C36-440B-A7F8-2B9D3FB38696}: NameServer = 192.168.1.254