XboX Doesn't go beyond the green startup demo!

Hi guys, i got an xbox today from a friend of mine to keep, coz it's dead. it was modded and my friend deleted some files in it, after that it only boots to that startup demo, nothing else comes up. how can i fix it, i don't know exactly what modchip it is, it is a 2005 may xbox, black color and has a samsung drive in the machine, 8gb hard drive, plz help me to get this fixed so i can finally play halo!