I once had that too, but it can only ALT+TAB if there is a different window open than the game. Try it yourself, open just 1 window and try to ATL+TAB. I think it would do <windows>+<D>
See if your <windows> key isn't accidentally pressed when you press D. When you experience the desktop again or it deselects a windows, press the L button, if you log off, it is definitly that <windows> key.
Or maybe the key is stuck??? or it gets pressed together with CTRL or ALT. Maybe your fingers are a bit wrong and use that button without you even noticing it? And like Gerryf said: a Macro or Hotkey program could be the problem. Or some hacker? Update all of your software and download and run the Malware section: http://www.geekstogo..._Log-t2852.html
Just gave tons of tips and possible causes, I hope you appreciate it