Could it be not enough RAM? I only have 512. Just a thought.
You can check while you're playing a file. Go to Task Manager (easiest is to hit CTRL+ALT+DEL and choose Task Manager from the dialog box), go to the performance tab look at the box on the bottom left [Commit Charge (K)]. The total is the amount that the computer is currently using, Limit is the amount of your physical RAM + Virtal memory size, and Peak is the most that has been used by the machine since the last reboot. If your Commit Charge Total (or Peak for that matter) is larger than your Physical Total (from the top right box) then you should add memory, if not, your problems may lie elsewhere.
It's probably a good idea to bump yourself up to 1G, I believe XP pro idles with about 200-300MB in use, so that doesn't leave a tone of memory for other things.
Also it's good practice to make sure that your Virus Scan is up to date and has an auto-updater installed to keep any nasties out.