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)

Change your display mode with the "Graphics mode setup" program in the Duke folder. Try overlay or surface.

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.