Search for the Norton 500. Patrick Norton who was one of the hosts of The Screen Savers on TechTV before the merging with G4 made a pretty damn good pc for roughly 500 dollars. The parts used will not suffice now, but with the time he made it (about a year ago), I'm sure sufficient parts will be cheap enough for that price range.
I really suggest you save up about another 650, though, and build a box.
Gigabyte - 7N400L - nVIDIA nForce2
AMD Athlon XP3000+
PNY Verto GeForce FX 5900se Video Card / 128MB / AGP 8x
Kingston Dual Channel 512MB PC2700 DDR 333MHz Memory
Western Digital 160GB
Diablo ATX Mid-Tower Case with 450Watt Power Supply
Ultra All Copper up to XP 3200+ CPU Cooling Fan
Sony DRU530A DVD Burner