It depends on how slowly its working. If its just lagging a bit it'd probably be fine to just give your computer a good clean out. That is, get all the stuff you've downloaded, put it on discs and then delete it from your computer. How much space do you have left on there? Because, with me, when i get down to about 1 gig storage left (haha) my computer (mac G4) starts working pretty damn slow. Also, little things like making sure you turn your computer off every night... and crap. Haha.
But if its working reeaaallly slowly you could always do a wipe of all the data... and start from scratch. It'd most probably work much better after that, as long as it isn't something else thats making your computer slow.
If you wanna do a clean wipe then back up all your data (more than once...)
I'm not sure of the exact procedure for wiping your computer, so if you do wanna do that you might wanna look it up. That manual above might have it in there.
Hope that helped?