Instead of disabling HW and stopping services, let's say for gaming, create a Hardware Profile.

Right click on My Computer - select Properties. Select Hardware Tab, then click Hardware Profiles.

Mark "Original Configuration", then Copy and then Rename to e.g "Game" Now you have created a menu that shows up when you boot your computer. Set the properties for your boot option in the "Hardware profiles selections" You normally would boot with the Original Configuration. Never delete your "Original Configuration" if you boot in another profile. Reboot your computer to see that this takes effect.

Now, to tweak the different services for a Game Machine listed in this document

http://www.tweakxp.com/article37047.aspx

Select each service to configure, right-click, select Properties, Choose "Log on" tab. In the lower part of this box select "Disable" or "Enable" dependent on which hardware profile you configure. If you configure for "Game" you choose "Disable" to free  up system resources.

Careful: You can also disable/enable different hardware in the Device Manager - "Device Usage" box according to what Hardware profile you boot on, this is only recommended if you have any hardware that causes problems for games, etc.

If you used XP "clean-up" on  your hardisk, then defragmented it, the PC would perform even better. Enjoy.