You could try editing your config crysis config file. Its in "my documents/my games/crysis_warhead/game.cfg".
r_Fullscreen = 0
r_Fullscreen = 1
To whatever resolution you require (I would suggest 800x600 to start with)
On another note, the latest ati driver wrecked my pc (bsods everytime I tried a 3d game and very unstable), so I had to go back to the ones that came with my card (Asus 4850) and all is well again. Worth a try?