Check if GUI.Window is open

Hello people!

What is the best way to make the windows in menu and deactivate another buttons meanwhile? Only when I close the current window I become able to click on something else. - this is the question

Im trying to figure out how can I check whether my window is opened or not. Probably it was discussed before, but due to fact that I am a newbee I couldn't find any discussion about it. I have my menu, there are a few buttons, I push the first button "start" - level loads; I push the second one "About" - window opens with an information about my game. But! While the window is open, I still can push the buttons under it.. they are active, laying beneath the window. What is the best way to make the windows in menu and deactivate another buttons meanwhile? To put it this way - only when I close the current window I become able to click on something else.