The screen turns black, a popup appears and in the center of the screen you see: 

"You may be a victim of software counterfeiting."

Down in the bottom right hand corner, you see:

"This copy of Windows is not genuine."

Any number of reasons why you might encounter this message and all you really want to do is get that scary message off of your screen.

Launch a Command Prompt as the Administrator and execute the following command:

slmgr -rearm

Reboot.

This is NOT a method for pirating Microsoft software nor do I encourage software piracy.  This command will only buy you time.  I believe the "counterfeiting" message will reappear after 30 days.  I do not know how many times you can run this command before Microsoft will come and arrest you.