First, I don’t know of any nVidia card that has DirectX 10.1 support, too bad for the new expansive nvidia 200 series owners. ATI 4800 series a fully compatible.
Second you must have Windows Vista Service pack 1 installed, not even the regular vista will do. Sorry.
Now you must have the game correctly installed and working, you maybe have noticed that there is a disabled DirectX 10 tab in the video options of the game, this trick will enable it as well.
Instead on running the game normally (double clicking the icon) right click it then select “Play Tom Clancy’s H.A.W.X DX10”, (You can also browse to the game’s installation folder and open HAWX_dx10.exe instead of HAWX.exe )
You are good to go, now you can enable DirectX 10.1 from the new directX 10 menu in the game’s video settings.
This has been tested and found working on my ATI 4890.