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.You can download the patch at Gamershell.
UT3 patch - fixes Windows XP 64 / Windows Server 2003 support
Posted on Sunday, October 14 2007 @ 21:21 CEST by Thomas De Maesschalck
Epic Games unveils a patch for the Unreal Tournament 3 demo to add support for Windows XP 64-bit and Windows Server 2003: