Check for spyware/adware first thing. Use an online scan. Trendmicro.com
has a good one. They also have an online virus scan (wouldn't hurt).
Next, nix everything you don't need that is starting up in Windows. You need to use the Startup Configuration Utility (do a search for it on htis site, as I am tired of typing the steps out).
Next, clean out your registry. Download TweakNow (http://www.majorgeeks.com/TweakNow_R...ner_d4134.html
). It's free. Run it and follow its directions.
Next, defrag your hard drive.
After all of that, your comp should zipping right along with no CPU worries.