If your Xbox 360 is red-ringing, and you’re looking for half a clue as to why, check out the AV Science Forum’s 3 Red Rings Xbox Diagnostic. It explains how to put your 360 in to diagnostic mode, and what the different patterns of flashing lights mean, such as “0001 power supply problem” or “0022 GPU Error / GPU Overheating”.
It didn’t work for me — my 360 went into diagnostic mode, but didn’t give me any codes (or at least, none that I immediately grasped). Still, it’s worked for others, including a friend of mine, so it’s worth checking out.