It could possibly be a corrupt EEPROM. Do you have an OLD backup of your EEPROM? (eeprom.bin)
If so, just rewrite your EEPROM using ConfigMagic, and you will be good to go. If you don't have a backup, you will need a new EEPROM file. You can buy these in the B/S/T, or, alternatively I have one you can just have that is XBOX Live safe, and has never been used.
It is a v1.1-1.6 EEPROM though, what version of XBOX do you have?
