Check the items listed below in HijackThis, close all windows except HijackThis and click Fix checked:
F2 - REG:system.ini: Shell=Explorer.exe C:\WINDOWS\Nail.exe
O2 - BHO: (no name) - {302A3240-4805-4a34-97D7-1645A0B08410} - (no file)
O2 - BHO: (no name) - {53707962-6F74-2D53-2644-206D7942484F} - D:\Programmi\Spybot - Search & Destroy\SDHelper.dll (file missing)
Then reboot and post a new log.
Regards,