How Do I Fix This Xbox 360?

It belongs to my 16 year old son. I am a single mom with not much money, he works at a pizza place so I am having him pay for any repairs but I am trying to figure out how to fix it the cheapest way we can. It has a red light, just one, not the red ring of death that I have read about. According to what I read than means that the AV cable is not working and he got an error code of E74. We bought an new AV cable and still get the same thing. We also borrowed a cable that we knew worked on someone elses and still the same thing. When someone else hooked up his original AV cable to their Xbox 360 it didnt work so we do know that his cable was bad. But what else can be wrong?

Suggestion:

1 red light = Broken graphics card, not AV cable (AV cable = 4), unless your son has the new XBOX 360 that just came out (in which case 1 could mean AV cable). I've had this same error and you need Microsoft to fix it