To play LOTRO in windowed mode, hold CTRL and press O. Click on the Graphics tab to open the Graphics Options. Uncheck the “Full Screen” box to play in a window.
Check the “Full Screen” box to re-enter Full Screen Mode.
You can also enter windowed mode by holding down ALT and pressing ENTER.
If you are running LOTRO in full screen, you can minimize it with ALT and TAB.