Epic Games have released a patch for the beta demo for Unreal Tournament 3 for those of you who are not able to run it (thanks Blues). If you have Windows XP 64-bit or Windows Server 2003 and you are receiving the "Modified executable code is not allowed" error at startup then you should replace the game's executable with the one from this patch.

Unreal Tournament 3 Beta Demo Patch (9.47MB)