I have just downloaded and installed Descent 3 from GOG, and when I try to run the game, it looks like it is trying to display the into movie but it is very dark and is frozen.
I have tried all backward compatibility modes.
Is it because i am using 64-bit Win7?
My hardware is:
Intel q9550
Radeon 4870x2
4Gig memory
I also have an old Descent 3 disc I tried before i got it from GOG that did the same thing.
I thought maybe the gog version had fixed for newer OS's.
I should mention I have tried OpenGL and Direct3d

You should try www.descentbb.net for help. There are many using it on Win 7.

Descent 3 works just fine with Windows 7, without compatability properties. Maybe it is teh Radeon card you use. I have nvidia all teh way because well nVidia owns all. What are your Launcher settings and stuff like that. Post them here and I will try my best to keep up with this thread because well Descent is an awesome game, and needs more players to keep the community alive.
But before you get into all that and finish reading this reply, open teh setup option in teh Descent 3 Launcher. Make sure that your video card is displayed correctly, use Direct 3D for your card. use the Auto configuration tools in teh console and make sure that they hardwares for the Sound and PC config are similar. The Processor and a few things in the system tab may differ from your configuration, but that is because the game is made for Windows 98! See how this works. If it is still failing, look threw the read me for the command line options and play with these a bit. Last but not least, install DX8 again. Yeah so what if you have DX9 or 10 installed, components of older DX versions are removed and or not that compatible with newer versions.

directX 8 or before are handled by DirectX 9a i think he should also try glidewrapper and use that
also if you are running more than one screen or in sli/crossfire disable them and run game as admin and windows 98 or 2000 for best results
My pc is even more likly to reject it
Core i7 980X
12GB DDR3 2133MHZ Corsair
Gigabyte X58 USB3 enabled motherboard
2x Geforce GTX480 + GTX285
Xfi Fatality pro
and windows XP X64 and 7 pro X64

http://www.descentbb.net/viewtopic.php?t=16700 is a thread about newer PCs with D3.

Have you tried Google?
It comes up with this: http://www.dateiliste.com/en/descent-3/26-descent-3-installation-updates-and-modifications/120-download-the-full-version-of-descent-3.html