You could have something causing a memory leak or it could, like you said, be caused by registry corruption. Try ctrl alt del to find out what is running that you can stop. Or run msinfo32 to find out all of the software programs that are running at any given time. Or run msconfig to check your startup programs, config.sys, autoexe.bat, system.ini and win.ini to see if you can find anything out of place.

I run 98SE but if I had a copy of 2000 I would go with that. I did a fresh install not too long ago and it made mine speed up.