A whole bunch:
Starting with Halo PC, you need WinRar to pull the maps from the .cab files (unless you do a full install)... Then you need a program caled Halo Map Tools (HMT) (v3.0 or later, since only new versions have extraction utilities), 3D Exploration (3DX) to convert the files from .obj (Lightwave object file IIRC) to a .3ds mesh, and then you need 3DSMax (any version i'd suppose, though i use 6) to import the new object.
I'm not sure where to get 3DX, since it supposedly costs $300, and there aren't many places you can find the beta version.
HMT can be found at Halomods.com
Winrar is at
www.rarsoft.comAny you need the full version of HPC, since the demo doesn't have single player maps in .map form, and ripped copies won't do, since HMT needs the patched version of HPC to work...