Now don't take this the wrong way, but, I'm a Mac guy. I only tell you that so you'll understand that I don't understand Windows stuff too much. However, my son does have a PC and he started to experience problems like that as well. Turns out he had a lot of "spyware" running the background that was slowing down his machine and making surfing the net almost impossible. He's running Windows XP Home Edition on a brand new machine with all the latest gizmos etc, etc. We ended up downloading a freeware program called "Ad-Aware" from Lavasoft.
Ad-Aware Download Page Try it, it can't hurt and it worked great for him.
Jonnie
(haha)