"We created a fix for the game not starting up on Windows XP 64 and Windows Server 2003 systems. This patch has not undergone the same amount of QA as the released demo so I strongly encourage to ONLY replace the UT3Demo.exe with the one from the patch if you are experiencing the "Modified executable code is not allowed" issue at startup."
Download:
http://www.gamershell.com/download_21433.shtml
Download:
http://www.gamershell.com/download_21433.shtml