Halo Custom Edition is a multiplayer-only stand alone version of Bungie's sci-fi shooter. Halo CE includes several new multiplayer features and options, the most notable of which is the ability to play user-created content. This add-on is unsupported and requires a Halo CD and Key to install.
Following these steps will get you playing online in no time!, without the annoying 'cd key is invalid' eg.
How to start playing online;
1.Run 'HaloCustomEditionSetup.exe', Install Halo with key provided in CD_KEY.txt
2.Once installation finishes, go to C:\Program Files\Microsoft Games\Halo Custom Edition
3.Run 'haloupdate.exe', let it download update and run.
4.Once update has finished, copy over 'haloce.exe' in the download folder to C:\Program Files\Microsoft Games\Halo Custom Edition (replacing original one, this enables you to play the game without inputting the CD.
5.Enjoy your fully working halo online.
I have followed this routine numerous times and it always works. :)