[Suicide Junkie]

Burn all your important files to CD.
Savegames, mods, etc.
Documents, bookmarks, desktop stuff.

Anything you don't have on CD.

Then, format your harddrive and reinstall windows, copy all your stuff back down, and reinstall your programs.

A new, good system (that I would buy) will run you only $500 Canadian if you put it together yourself.
Rebuilding your system as above will cost you $0, and no more than about 4 hours unless you've got tons of files you need to backup.