Released for free by Bungie at some point, as both datafiles (for running under Aleph One) and as the original CD-image (.iso file).
The game installer is 16 bit executable, so you can't install it on 64 bit Windows as support for them was dropped. Game itself is 32 bit, so if you manage to install it, playing it won't be an issue on 64 bit system.
About Marathon 2: Durandal
2008-03-27 (updated 2008-03-30) #