We never worked out what it was, but a few months ago both my sister and my Dad watched something online that totally fucked their computers. I mean totally. I was employed to "fix them" but after a lot of huffing over the command prompt, I got no where with my sister's and only as far as you have with my Dad's - blue screen, go into the hard-drive to reinstall/error check, and it crashes 50% of the way through. In the end we took them to a computer shop - and my Dad's was miraculously alright when we turned it on to demonstrate. I think it suddenly decided to do what I was telling it to do, and error checked. My sister's had to be taken apart and rebuilt.

So unhelpful, but basically I'd act fast.