Did you try a clean install of the game? (uninstall, reboot, make sure the game folder is gone or empty, shut down all non-essential programs, and reinstall)
Does GamersGate's download/game manager say the downloaded file is ok?
Disabling the real-time portion of anti-virus programs is a good idea when installing large programs from a trusted source, but the anti-virus program itself shouldn't cause a problem, unless something is seriously wrong.
