You can get Win98SE (Second edition) at any computer show, and it's really cheap. It was probably the most stable system until XP came out. If you're set up OK, it runs fine and you don't get the "blue screen of death", unlike the previous versions of Windows. DO NOT GET Windows Millenium, it's a horror when trying to run games. It's "Lock up city" and a real pain in the butt. You need to check on what type of video card you have in the computer and see if it meets the spec's of the game. After the CPU and motherboard, the video card is the most expensive item. Then, check on the kind and amount of RAM that is installed. Just remember, if your system just meets the minimum requirements of the game, it will run, but you"ll have to reduce or turn off so many features that it might not be much fun.
If you shop at computer shows and build it yourself, you can get a real good "one generation behind" gaming computer for around $600. Most of the money will go for the CPU/motherboard and video card, you don't want to skimp on either.
BRG,
Jon