There were Ctrl+E+C (comment) and Ctrl+E+U (uncomment) in older versions, or Ctrl+K+C and Ctrl+K+U.
But in Visual Studio 2012, I can’t see key shortcuts:

How can I enable those shortcuts?
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Keyboard accelerators are configurable. You can find out which keyboard accelerators are bound to a command in menu Tools → Options on the Environment → Keyboard page.
These commands are named Edit.CommentSelection and Edit.UncommentSelection.
(With my settings, these are bound to Ctrl + K, Ctrl + C and Ctrl + K, Ctrl + U. I would guess that these are the defaults, at least in the C++ defaults, but I don’t know for sure. The best way to find out is to check your settings.)