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I've had the same thing with a friends Lappy. It was riddled with virus's.
Get yourself an IDE/SATA - USB drive converter cable, I got one for about a fiver from ebay, and it truly is the most useful bit of kit you can own.
Pop your drive out and plug it in to another PC with up to date security.
Then you can do what you like with it, back-up/delete whatever you like, virus scan, cleanup, defrag.
If you're lucky a decent virus scan will sort out 95% of your problems allowing you stick your drive back in and update your AV and Windows.
If you're unlucky at least you'll have an easy way to back up your stuff before you reformat and reinstall.
I've used this method several times on a variety of problems and computers and it really is the most painless way to go.
Hope that helps.