Can't uninstall Mass Effect?

Try troubleshooting using the official microsoft web page, as this is not a bug in Origin.

http://answers.microsoft.com/en-us/windows/forum/windows_7-windows_programs/error-codes-2503-2502-while-unistall-or-install-a/6a996faf-7e28-40bd-b5e0-02ab17dfcd88

Hope this helps :D