Replying to Speed issues

nearly 3 years ago i got a powerbook g4 for college and have gotten a ton of use out of it since. this past semester ive noticed its starting to slow down. this summer i bought a 250 gig external hd to move my music (about 25gig). i have probably 20 more gigs of video and a few programs like photoshop that i use occasionally. i was wondering if theres any quick way to defrag or whatever you do with windows to just get rid of all the crap ive probably accumulated over 3 years of reckless downloading and shit. ( ive yet to get a virus or experience any freezing. damn i love this thing.)