What are you doing on the computer when it runs slow? Have you run a virus checker recently? How much free space have you on your hard disc? How much system Ram have you got? Have you run Spybot or Adware?
Have a clean out in your Temp and Temporary Internet folders.
Throw out the clutter , any unused programs might as well be uninstalled , if you have too many desktop icons , delete as many as you can (you can still access them through`start`programs).
Have a look at the systray, most people have to much running unnecessarily , get rid of media players (quicktime,real player , winamp etc)for a start , most things if right clicked have a `preferences` , you can also have a look at your startup , dont take out anything your unsure of.
After clearing out temp folders and uninstalling etc you might have reclaimed a fair bit of space back , so it would be worth while running defrag again .