Wayne's technique is about the only way you're going to pull this off if you're using 'real' windows.
If you've noticed in many games which 'take over your screen', the developers actually recreate Notepad within the game if text editing is required - everything 'Windows' in a well formed Full Screen game has to be reverse engineered and recreated within the full screen environment.
Even with Wayne's idea, if your app takes over resolution as well (as many games do) then the switch back to Windows mode is going to look, well, weird. You've probably alt-tabbed out of a game at some point and seen humongous icons on your desktop - that's the effect.
Try the suggestion, but consider how you might improve upon it by creating your own Virtual Notepad which works within your game.
Happy coding!
Ned