I did most of these yesterday and there is quite a difference. Takes 10-15 mins.

http://www.mobile-pedia.com/?q=TechArticles/SpeedupWindowsXP

In this we could post additional advices that makes the PC run smoother....

you can also speed up XP by disabling all animations for menus and folders.
reset all visuals to classic windows. this will speed up your machine but it will also make the visuals look like you are using win95 or 98.

also when you use msconfig there is a popup that appears each time windows starts saying that you are using msconfig to diagnose problems, tick the check box to disable the popup.