Over the last weekend, Bungie released a new patch for the Windows version of Halo, v1.08. (The original announcement from Bungie's site is viewable here.)
On top containing several fixes for server crashing security bugs, this patch notably removes the CD check, as a good will gesture to their dedicated fans. A valid and unique product key is still required to play online though. Unfortunately though no effort has been made to combat aimbots, as they consider the game too old to be worth the effort.
v1.08 clients aren't back compatible with servers running older versions.
Bungie also released versions of the patch for dedicated servers and clients of the Halo Custom Edition, a version of the game that allows user-created maps.