Epic's Unreal Tournament III demo hit the internet late last Friday, and the release has been quickly followed by a patch. The update is for a Windows XP 64-bit and Windows Server 2003-related error.
"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", the developer's explained in a forum post.
The patch can be downloaded via the CVG source.