Help With Cd key for Halo

[Pucci Luda]

Hey guys i got my halo along time ago and i lost my cd key so when i installed halo on my new comp i had to find a cd key to use, but it wasnt my original one that came with it so now when i go and try to get into an internet game it says your cd key is invalid

can you guys help me

is there anything i can do

i already have the no cd patch and it works great

thanks