Crash on launch

[The Bard]

Pardon if I appear to be necroing a thread here (no pun intended... or is it?) but I imagine it's better than starting a brand new topic on the exact same issue.

Before we get into the nitty gritty, some system specs:
Windows Vista 64bit
Intel i7 920 CPU
6GB DDR3 RAM
Radeon HD5870
Gigabyte GA-EX58 Extreme mobo

I bought the game on Steam as part of the Aspyr Complete Pack over the Christmas special period so I can't complain about 'not getting my money's worth.' At the time though I had a NVidia 9400GT (as my last GFX card was a dud and the mobo has no onboard video) and Necrovision worked as fine as expected on such an underpowered card.

Now, however, with the HD5870? I get the generic error message, same as these lads. The card has been the only alteration to my system so I imagine there lies the heart of the issue. I have the readme for the 1.2 patch in the Steam files so it's likely been patched as part of the main installation. The drivers are current, including the fix for City of Heroes (Necrovision didn't work before or after the fix).

If a solution can be found, I will be grateful as I play the only WW1 alt history FPS I am aware of since Ironstorm (Darkest of Days doesn't count as 'alternate history' )