you could buy a cheap tv tuner and connect your xbox to your computer only problem is sound (at least i haven't figured a way to make it work)
Just get an adapter that has two rca jacks on one end and a mini headphone jack on the other. Pretty common, Radio Shack etc. Plug into sound card line in, stereo, headphones, whatever.
My cheapo ATI tuner I'll have to play around with though for playing a console game on the monitor. Picture didn't look very good on the normal quality setting. Looked better on the high quality one, but then the video lags enough to be annoying.