Play RYL2 in windows mode

Start => Run => Regedit

HKEY_LOCAL_MACHINE => Software => GamaSoft => MP-Client

For Windowed mode

:Alter the values Height and Width to a different aspect ratio as your Desktop resolution

Example:
Desktop Resoltion: 1024(width)*768(height)
Valid Windowed Resolutions: 1024*760 - 800*590 (or something else thats almost the full screen res minus a few pixels).