To close a window: Make sure the window is selected (hold down the ALT key, then repeatedly press the TAB key until the window you desire is selected). Then (once the window is selected), hold down the ALT key and pressing/releasing the F4 key.
Another method: Make sure the window is selected (hold down the ALT key, then repeatedly press the TAB key until the window you desire is selected). Then (once the window is selected, press ALT and the SPACE BAR. The selected window's "system menu" will open. From there, you can use the up/down arrows on your keyboard to select/highlight minimize, maximize or close. Once your choice is highlighted, press enter.