Nope.
If you've deleted it off of your machine (too bad, 'cuz it IS recoverable while installed, even if Halo wasn't working
) and its lost its gone now.
I doubt VERY much that anyone would "give" you a new CD key: How would they know 1) you haven't done this with them before, and 2) you're being honest with them, and not just trying to scam another key?
Frankly, from their perspective: why don't you just shell out the $20 and get a new one?
(And I'm STILL looking for the lowest $ for an actual new Halo CD key for LAN gaming.... Please PM me if you have any source for @/under $10....)