I've been using this copy of Windows 7 for several months without a problem until it recently decided it wasn't legit. I know there's the occasional genuineness checker thing that comes with the auto-updates, but I usually look through them before I download and I didn't notice anything like that. I'm guessing that is the source.
I had this problem with XP a while back and there was a little program that fixed it, but I can't seem to find one for 7 that works. If anyone has some solutions besides re-installing Windows, I'm all ears.