Actually, I have too many suggestions.
There are a number of things that could be doing this. Primary suspect would be a piece of evil software that has somehow gotten into your machine, but I'm sure you've got the tools to check this out.
After I threw Norton's various tools at this (along with Ad Aware), or the equivalents thereof, I would then run a disk check.
Norton has an online virus and security checker. It takes a long time, but it's free.