Chances are, if its a prebuilt, you can't overclock it. CPU's not that bad. I'd throw in another 512MB of ram just for a bit of a boost. You can get a stick of 512MB for $30 so it wont cost much.

You could try reformatting. That should definatly give you a bit of a boost.

I wouldnt recommend anybody to "try" overclocking without really having an idea what hes doing - do yourself a favour and read about it first - read A LOT

@Stardestroyersarecool:
I dont think theres a lot you can still do with that machine, even an Oc-pro couldnt squeeze out more than, say, +10% at best. The videocard is just too weak and any upgrade would be a waste basically, better save up some money for a new rig - but thats me
btw what kind of board is it, AGP or PCI-E ?

Reformatting is wiping all data from the hard drive and reinstalling your os and games..
you can speed up your old comp by using the task manager to see how many processes your running. if your running say 35 or more processes, you should try to bring that down some.

Processes are apps that are running on your comp at that time.
Defragging will speed it up a little too.
Any secondary programs running at start-up, that can be turned off, without effecting the games or vital systems, shouls be turned off.