For months I had trouble with my new PC's performance after I had used it for a while. One problem I found was spyware, which can be found in programs such as Kazaa. Use ad-aware (free program) and it will help you a bunch. Also, update your anti-virus. I was shocked when I found viruses and malicious scripts running on my computer. Most of them being, ahem, viruses from adult content (watch out for that pamela andersone and tommy lee video, its a stinker).
After working on my computer for an hour or so, it was purring like the day I bought it.