I solved the problem a while a ago like this:
My main form was some kind of splash screen, which had a few basic options to start the "tools", which where the windows in which you could actually do stuff. When such a tool was selected the splash screen got hidden. You could create as much "tool" instances as you wanted from the menu of each of the tools (so basically there was a New many point which had all the options available in the splash screen), and if one tool closed, it notified the splash screen, and if no other tool was visible, the splash screen reappeared.