ccleaner should work fine as a registry cleaner. CCleaner - PC Optimization and Cleaning - Free Download
After that use Disk cleanup. (rightclick on your C drive -> properties and then Desk cleanup)
Defrag the C drive.
then type in "msconfig" as a RUN command and go to the startup tap. in there you can deactivate useless startup applications.
you could also just re-install windows XP.
---------- Post added at 08:20 PM ---------- Previous post was at 08:17 PM ----------
ohh. btw: How old is the system and what programs do you use.
also do you use the computer for gaming?