Last week, the computers of my dad and a daughter caught a virus that looks just like Windows own security software. You can tell virus from the real security software because (1) it pops up so many dialog boxes and balloon alerts and(2) it asks for money when you agree to "eradicate" the virus.
(Both computers run anti-virus software, yet were infected. Perhaps the anti-virus definitions were not updated? Both asked me where the virus came from; I don't know, but I suspect they clicked on a link in a Web page.)
I found the following steps work to remove the virus:
1. Download the free version of AVG from here <http://free.avg.com/ca-en/download-free-antivirus>, install it, and then allow it to do a full scan. The scan can take several hours, and it will erase the virus.
2. The virus damages Windows Defender, and you cannot reinstall the software. The solution is to rollback Windows by following these steps:
a. Restart your computer.
b. As it starts, keep pressing F8 repeatedly.
c. When the boot options screen appears, select Safe Mode.
d. After Windows finishes loading, start System Restore.
e. Restore the system back to a date earlier than when you first noticed the virus, say a week or two earlier.
Windows will probably reboot the computer, and then Defender should be back to normal. You may need to reinstall software removed by the rollback.