Weigh up the value of time expended attempting to troubleshoot such a problem with the minimal effort of backing up important files and then wiping and starting afresh. In the majority of cases where there's evidently a fairly hefty problem with the computer, the latter option is always the most time effective.
Note that if you do go for the latter option
DO NOT attempt to backup any software (and I mean NONE, not even installers !). Only backup actual files you know (images, word processing documents etc.).
Oh, and perhaps take the opportunity to step up to Windows 7 whilst you're at it.
