If programs won't run properly, Windows crashes often or takes more than a few minutes to boot, or you've got a malware issue you simply can't overcome, that's when you need to cut bait and start over.I've reached that point with my system. So I'm making lists of all the essential programs and utilities I'll need to reinstall, backing up all my important data (including easy-to-overlook stuff like address books and iTunes folders), and syncing as much stuff to the cloud as possible (for ease of retrieval after the system wipe).If you feel like this is beyond your capabilities consider hiring professional help. It'll cost you a few bucks, but certainly less than a new PC would.
