Alright, I'm going to start by saying that I don't have the GoG version of the game. I do, however, have both the original retail and the free version from fileplanet. Problem is as follows.

When I try and enter the "Configure Squads" menu, the game bombs to the desktop and throws up the following error dialogue:

"FatalError
Can't open file ENTMODELS/MANAGEMENT/CC_POWERARMOR_MANAGEMENT.MDL"

I'm not sure, but I think it's down to the Terradynes, as I can open the configure squads menu for mission 1, but mission 3 bombs out. The obvious difference being the addition of the light Terradynes. This, of course, is a pain, as I can't configure my squads for roles or their equipment.
Thing is, I've no idea what's causing it. I've tried retail on it's own, I've tried retail + Dark Conspiracy and I've tried free version on it's own. I noticed that there was a discrepancy in file size for my graphics.sdf file between what was installed and what was on the disc, so I copied that over, that didn't fix it.
The only thing I can think of that may by causing it, is the fact that it's installed on my E drive, rather than my C drive.

System specs:

XP SP2
3Ghz P4
ATI HD3850 (latest drivers)
4Gbs RAM

Any help would be greatly appreciated. :3

Found this in google, same error, but it's dealing with custom missions, not the original campaign.

http://www.massgate.net/read.php?123124,215759

Don't know how much help that'll be.