Moved to Top Threats.
Antivirus engines generally wont install with malware of that nature installed. Try to intitiate System Restore to go back to before all this started. If necessary reboot into Safe Mode to do that by tapping F8 repeatedly while booting up and then selecting that on the menu that shows.
If this ever happens again, never click on anything at all as a simple click on even the 'X' to dismiss it will activate the ransomware, simply power off and then try System Restore in Safe Mode.
Malware frequently checks for the presence of antivirus software and disables it, and also disables System Restore. It can do this only because it's a Trojan, it arrives pretending to be a legitimate program (often a Flash or Adobe Reader update, or something similar). I assume that it would also block the installation of anti-malware programs, as here.
There is a way to get rid of it using an anti-virus program on a boot drive. McAfee has something licensed from Microsoft which isn't freely available to Home users. Sophos have something you can download, and so do Kaspersky and Microsoft. (Okay, these aren't in-house solutions. But they should work.)
The Sophos tool is specifically designed (they say) not to clash with existing antivirus programs, and is supposed to be a second opinion. You can download it from here onto a flash drive, although whether the drive is then bootable is not certain. Kaspersky's Rescue Disk 10 can be downloaded to a USB drive from here. Microsoft's Windows Defender Offline is available from here. In each case you should use a new flash drive - WDO will format it before downloading to it, so anything already there will be lost.
I thought I should try out the Sophos program to see how it performed. It's a pretty big program to download, and the Licence agreement is eye-wateringly legal. It downloads a whole lot more before it's ready. It can be saved to a USB drive, but as I said above I don't think this counts as a rescue-disk version, it doesn't appear to be designed to run at boot time. And the scan is taking forever to run, it looks like an extremely thorough full scan.
I've got Wagner's Die Walkure on the radio and I'm not sure which is going to end first, this scan or Valhalla.
(Threats found : 0. Percent complete : 2).