Could be so many things. But sometimes, and I have had this problem with Comcast in the past, they throttle your connection. IDK Why, but it happened to me when I first signed up them. I had a 6meg pipe, and I was downloading at like 23k a second.
I called them (in a huff) and told them to return all my money I had paid during set up, because I wasn't going to pay $$$ for a 6MB pipe while I'm only downloading at 23k p/s. The next day, everything was fine, and I was zooming happily across the highway.
Of course, it could be any number of reason's why. You'd need to provide some more background about your problem. When did it start? What changes did you make to your PC before this problem appeared? Any other symptoms show up on your PC at the same time this one did? And what have you already done to try to resolve it?