I just worked on an older Dell desktop that was extremely slow running XP-SP3. I had set that box up so I new how fast it should have been.
I disabled everything on the Startup Tab in msconfig that was not absolutely necessary, as others have said, and then ran CCleaner.
Against typical, it was when I ran the registry cleaner that the machine snapped back to life.
I'm pretty sure they were victims of an unattended iTunes install and incorrect uninstall. That is what the majority of keys-to-nowhere were about.
I will do a clean install for them the next time I'm visiting on a rainy day! I agree fully that is always the best way to freshen up an XP box.