There are programs that try to artificially slow your processor down enough for DOS games to play. One is called moslo I think. Generally they don't work very well. It's hard to slow down something enough that's a couple hundred or thousand times faster than the original programmers of the game expected. Even if the slowing program works, chances are your sound or video card isn't going to work quite right in an old DOS game. Programmers made assumptions about where in memory your hardware lived and such, and often it doesn't live in the same place as it used to. XP doesn't support DOS very well anyways. It has some crappy DOS emulation that it uses instead of real DOS. You'd almost have to have Windows 98 or earlier to play a DOS game well I'd say.

Best advice is to use an old computer. Or buy one if you don't have one. You can get a 486 for less than $100 in various places. EBay for example.

EDIT yeah Bleys posted while I was etc.