Just want to pass this information along because it looks convincing and I would imagine a lot of people are being taken by it. I've been in the computer business for a long time and I don't like seeing people being robbed.
The virus is called KardPhisher. If you reboot your computer and see a popup that looks like this you have been infected.
Once you reboot, Kardphisher asks you to reactivate your copy of Windows, citing piracy issues and telling you that another user has activated your copy. Though it assures you that you will not actually be charged, it asks for credit card information. If you don't enter the credit card information, Kardphisher shuts down the PC. The Trojan also disables the Windows Task Manager, which makes it more difficult to shut the malware down.
Here is an article about it on PCmag
http://www.pcmag.com/article2/0,1895,2147608,00.asp
Please feel free to pass this around to others.