What I've found works well is to use Hijack This! in safe mode to systematically remove any unusual applications. However, this requires a thorough knowledge of what is and isn't "normal" for the OS in question as well as a flair for spotting the bogies hiding among the friendlies - disabling the wrong things with HJT! can result in an unbootable computer!
So hat I usually end up doing is rebooting into Safe Mode, launching HJT, and then tracking down each and every autoloaded file by following the directory path it's in and checking the EXE in question for version info and other clues. If it looks suspect I make note of it, kill its entry in HJT!, reboot, and check to see if it restarts on its own. If it does, I force a rename on reboot (either by creating a WININIT.INI file or using the PendingRenameOperations Registry key, depending on the Windows version) to rename the file, reboot again (ignoring any errors about not finding the filenow), and remove the renamed file.
Of course this is awfully complicated and potentially risky if you lack the expertise, in which case I suggest seeking out some help.
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Deja moo - the feeling you've seen this bull before...
"Political Correctness is a doctrine fostered by a delusional, illogical, liberal minority, and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a turd by the clean end."