do what tweaker said, disable any programs and services that you dont need using Run->msconfig. I would also reccommend using Tune-Up Utilities, it can find errors in your registry and things like that. Also defragment your hard drive often and use disk cleanup.
Im currently testing how much of a difference these things make, i got a score of 3965 on 3dmark03, and i defraged and did a bunch of other things to the computer, i'll let u know how it turns out.