Consider Stepmania for the PC. It's an open source DDR clone, so it's free. It doesn't have quite the polish of most commercial DDR games, but it lets you create your own dances from mp3s. I'm not sure if you can do that in commercial DDR games for XBox or not. I've made several dances from video game tunes from vgmusic.com (I prefer to use midis converted to mp3s, since they have more consistent beat rates).
The hard part is finding a PC-compatible dance pad. Your best bet in this regard is to buy a PS2 dance pad and buy a PS2-to-USB converter.