Windows 7 Keyboard shortcuts switch unexpectedly
Win+M and Win+D have very similar results. Your D may be remapped to Win+D.
I have seen many times various modifier keys getting stuck. If there's a lot going on on your computer, keys can become stuck, not physically but mentally as far as the compter remembers. It remembers you pressing the key, but missed the point where you released the key, so it thinks it's still pressed.
Press each Alt, Ctrl, Shift and Win once each to reset their state. I've also had to do the same for Arrow keys in the past, particularly when playing games.