for the techies out there

what would be making my computer's fan be running constantly. Everything slows to a crawl when this kicks on. The room isn't hot. It's not dirty (but I'll be buying a can of compressed air anyhow). It seems to do it when I've got several windows open.

This computer is barely a year old.I don't want to have to replace it but from the way it's acting it's on the verge of something bad.

It has the free AVG on it which runs daily and I do ad aware weekly. nothing but cookies keep coming up