I searched for question similar to this, but they all mentioned error messages in Task Manager. I'm not getting any.
I could no longer open a new instance of either Firefox or Chrome after exiting them earlier by clicking on the X in the upper right corner. Opening Task Manager I found I had two instances of Firefox running, and eight of Chrome.
I tried using End Task and End Process Tree to kill them, and this got rid of half -- I still had one Firefox and four Chrome processes that could not be deleted no matter what I did. But no error dialog came up, the End Process command just silently did nothing.
I ended up having to reboot, and even that got stuck, it was showing "Shutting Down..." for perhaps 10 minutes and I had to kill the power. On powering back up, I expected the message "Windows did not shut down properly" but instead it went straight to the animated Windows logo.
This happens maybe twice a week to me, and usually associated with one of my browsers.
I am running Windows 7 SP1 64-bit with 16 GB of RAM.