Follow the tips directly after a clean install of Windows XP
Defrag the hard drive. Even if XP tells you that it does not need to, do it anyway. It has much tolerance for what "needs" to be done and what should be done. After all, if it was really efficient, there would be no need for you to read this page.
Defrag again after installing your applications and games. After that, there should be little need to do it again for awhile unless you delete/uninstall/reinstall a lot of stuff.
Read all steps here - Super Tweaks for Windows XP by Black Viper
This listing is by no means complete, but it does hit on the high points.
