I can hear the loading/title sounds etc, but can't see anything due to a bright green screen.
Anyone have any ideas what is causing this (can you get a refund, if it can't be fixed)

Windows 7 Ultimate x64
HD 5870 (latest CCC)

EDIT:

Forgot to mention it also screws up my desktop resolution every time i quit the game & i have turned off all CCC advanced 3D features.
Ironic that i have loads of other ancient games installed without one single problem & the first time i purchase and install a game from G.O.G a major headache ensues, i thought their service was supposed to eliminate all these sorts of issues.. a pretty poor experience for a new customer (a good job i tried this before i purchased the other 4 games on my list)
Apologies for the rant (better out than in)

Thank you, i tried what you suggested (in fact i tried every option) unfortunately it did not solve the problem, i installed the Ed Duke 32 thing 7 it works (intermittently) and the high res texture pack (hoping that these, would fix whatever rendering etc issues are going on) but that causes utter chaos.

Think i will chalk my flirtation with G.O.G up to experience (admittedly lost $5.99, but at least i've learned a lesson)
And as for the "community" a new member posts a problem & 15hrs later there is one response...

So long.

You can always try GOG support. Perhaps DOSBox is being weird on your computer.

I have the same graphics card and OS as the original poster and I had no problems running the game in either DOSBox or eDuke32. I'm not sure what could be causing the glitches.

Generally with Windows 7 you want to be sure to run games as administrator, and you might also want to install to a location other than program files (I did both of these with Duke). Not sure if you tried those already.

If the problem persists, definitely send a report in to GOG support.

Use eduke32.