how do I fix my old xbox it flashes red and green?

This is generally referred to as the "Christmas Lights" or Frag error, and it indicates a serious system problem of some kind, frequently hardware-related. Since the prblem could be any number of things, it's difficult to give specific advice, but here are some options:

a) open the case up and check to see that everything inside is properly connected. If you're lucky, a component has just come loose. If you're really lucky, it's just the power cord.
b) Install a modchip and/or replace the drive and or flash the BIOS. Check here for information about how to do that
c) nail it to your front door for a Christmas decoration - red and green is so festive.
d) Gently adjust with a shotgun.