Computer Slows Down - Say 100% CPU Usage

This is a fairly recent problem. I have a Dell computer about 4 months old. Here's a description of the problem. I would appreciate any assistance I can get. I was going to call Dell but I've found I get better help here than Dell! My CPU will start to get louder than normal (it sounds like it is working overtime in there). At the same time, IE freezes up and pages won't load anymore. I can still use email and MSN IM normally, as well as other programs. When I check the "windows task manager," it says the CPU usage is 100%, when normally it's around 2-5% It will do this for up to an hour. Even if I reboot, after it has started up, it continues do this same thing. After it finishes whatever it is doing, CPU usage goes back down and IE works normally again. I'm afraid it might be some spybot program or something that is taking over. I can't figure out what is wrong. But I know this is a good place to come for