Oh man...tell me about it.
Luckily, I too only had to reinstall a few things. (Even reinstalled my sound card drivers straight from disc) I about freaked out.
I did uninstall Avast and reinstalled it, so fortunately, it updated to the correct defs. But boy oh boy, there for a while I was like "what the heck?"
Anyway, thanks Avast for fixing this monster ASAP.