Since the 1.08 patch also removed the need to have the CD inserted while playing, the CD Key now has become the sole characteristic that represents a "valid copy" of Halo.
The CD contents are copyrighted, simply because they're "published content," but I'd bet that no one, not even MicroHard, would go after someone that made a copy of someone else's CD - SO LONG AS THAT SOMEONE HAD A VALID, LEGALLY-PROCURED CD KEY.
I mean, they've been shipping out free replacement CDs for 4 years as it is.... (Which they no longer do, BTW.)
So beg/borrow a Halo CD from someone, ANYONE!, and get on with it....
FYI: I would NOT trust a downloadable image from anywhere/anyone, as very few virus scanners can check inside an ISO image.... You get my drift here??