So I started having this problem a week ago and could'nt figure out why. I tried playing COD4 online and as soon as it reached the COD online menu the screen would turn black and return to the mein menu (XBAR).
I thought my disc was scratched but then I tried MW2 and it did the same thing as soon as the game loaded.
So I tried serveral things (including holding power to access secret menu) but none of that worked.
Then I found this forum and read about how some people deleted utorrent from their computer to fix the issue. I immediately remembered that my utorrent app on my computer upgraded to the latest version around the same time the problem started.
So I walked over to my computer, turned off uTorrent (by righ clicking and exiting) and tried playing COD4 again and WHAM!, worked like a charm again.
So please give this a try if you are experiencing the same problems and have uTorrent installed (or any other programs that you may have recently installed or upgraded). In my case I didn't have to delete uTorrent, just exit out of it.
Hope this helps!