DF United if you could fill in the following gaps, I'm sure we could be more help.
CPU Type/Speed (e.g PIII 600MHz)
Exact type of graphics card:
Operating System:
ALl that aside a three year old computer is still just that, a three year old computer. The first thing you should do is ensure that absolutely nothing which is not absolutely essential is using your PCs resources. Quite often you will have lots of stuff running in the background, these pretty much all have to go.
If you have a cd for your OS then I suggest doing a fresh install unless you have done this already in the last couple of months. Finally it might be worth considering a bit more system RAM, I know you said you really didnt want to buy new gear, but I think a bit more RAM will make for much better performance.
Good Luck:)