You sure it's not just some process that should have ended that hasn't?
When it slows down again do CTRL ALT DEL and take a look.
Firefox can be gay sometimes and not close after you exit. If you have firefox and it's running and it's using a stupid amount of memory just close it and re-open it.
Also check for scheduled stuff, you may have a background antivirus scan running or 10 programs updating at the same time or something daft.
I've not had any slow down issues with XP and I've left that for weeks without rebooting whilst using it for everyday stuff - internet, email, games etc.