Game-Playing Problems On Compaq 5320

I have a Compaq 5320. We used to be able to play games on this machine but now any game you try to play - from "House of the Dead II" to card games with simple audio and animations don't work. The symptoms - the games all start fine but after a minute or two of play the sound goes (becomes "echoey"), the video slows down and the mouse becomes very sluggish. If you can hang on, the game will pick back up. It is important to note that all this started before we were online. I have downloaded updated drivers for the audio and video to no effect. Could this be some kind of buffer problem? Could XP Service Pack 1 have caused this? I am ready to wipe this thing and use the Restore disk to reset to factory condition. Help