Try closing Steam (right-click in tray > Exit) and then finding the shortcut. Right-click > Run as administrator.
After Steam has started, run MW2 Liberation as administrator. If it still doesn't work, I want you to do this:
Open up Notepad. Copy and paste this:
Click File > Save and then choose All Files from the Save as type. Save the file on your desktop as admincheck.bat and then double click on the file. Tell me what it says.Code:@EcHo off net user %username% >> temp.txt find /c "*Administrators" temp.txt > NUL echo ---------------------- if %errorlevel% equ 1 goto notadmin echo You are an administrator! goto end :notadmin echo You are not an administrator! :end echo ---------------------- del temp.txt pause