Here is a quick fix I did.
I made a batch file with the command:
taskkill /im construct2.exe /f
saved it and put it on my windows 7 taskbar.
Try to close it normally before you use that.It is not a replacement for the close window. It is really handy to not have to press Cntrl-ALT-DEl , find C2.exe, and task kill it, confirm, close task manager , nearly every time I close it. If it freezes I just click on my batch file and it closes it quietly.
I have noticed it more often with Steam, and Desura. Especially with Steam if I immediately go from chatting with someone, change focus to the C2 window, and try to hit the X to close it.
Maybe this is just a coincidence though. I have had it freeze like this when neither Steam or Desura are in memory.
Thanks — I did not know this was a common thing. I was blaming my computer, and Steam+desura. :)