Just curious, what games have you ran on your box? I had an old Emachine box with a athlon 3500+ single core 1.9ghz with 1 gig of ddr and a nvidia 9500gt card and I could run Crysis in low detail with no lag or anything. As cheap as keyboards are these days maybe you should spring for a new one.
I noticed you wrote that you were using a PS/2 connection, you could try changing to USB but really it should be fast enough..
You could try messing with the settings highlighted in the screenshot as well as giving your machine a general clean using something like this free app: CCleaner - Optimization and Cleaning - Free Download