Re-installing the operating system.
I do not see it that way at all. If you defragment the drive and remove the adware-spyware and other crap it should never slow down at all.
Also, you positively MUST disable stupid services that are not used. In order to do so correctly, see this site:
Services Guide for Windows XP Go down to the Services Index click on the first service and follow directions until all services have been set up properly.
