A new install of XP, if you'll forgive my language this one time, leaves behind so much S**T. I allways run a reg cleaner right after doing a fresh install along with my little trusty friend> crap cleaner. Find junk files is another good one to run after that also. Next go to www.blackviper.com(yes he's back!!!) and check on the services you can shut off.
All these things serve to rob your system of top performance. So get the programs and run them and see how much better it does.
Here's the links>
find junk files
services/processes you can turn off
Mine is in Russian so you're on your own there. It's the only one I trust.
But here's the name of it anyway. NBGCleanr.
So there you have it in a nutshell. Run the programs from safe mode and they'll do a vastly superior job than runing from normal.
And do be careful about what services you shut off. You can lock yourself out of your computer.