I would imagine your problem (unless it is overheating, but then the problem would show when you play other games) would reside in the video card, since 512mb of RAM should be enough to handle it (it was released in 2002), and you have plenty of hard drive space open.
You can certainly try disabling any applications or unneeded programs running in the background, and if there are player options that allow you to change from best graphics, etc., to best performance, to adjust these. If you have not done routine computer maintenance (including scanning for malware and defragging), that might make a difference. Lastly, be sure you have the latest drivers for both your video and audio cards.
Edited by jgweed, 04 March 2006 - 11:44 AM.