Basically, to get onto live again, you will need a fresh (unbanned) EEPROM. This is basically the serial number of the xbox. It can be replaced by buying a code off the internet and then loaded onto the xbox. I believe you can get codes off e-bay (subject to usual risks) or http://forums.xbox-scene.com in the buy/sell area.
Once you have loaded the code (see a tutorial on www.xbox-scene.com ) then live should work as normal again.