Q: Removing a virus? Mackkeeper?

Nothing any of these windows claim is true. Every single one of those messages is to goad you into coughing up money to crooks. The ID number means absolutely nothing.

Restart the Mac and hold the Shift key to startup in Safe Mode. That will prevent any of this garbage from loading since only minimal services are allowed to load in Safe Mode.

Then download and run MalwareBytes for Mac. The adware makers have been getting very nasty lately. This stuff used to be fairly straight forward to remove (such as simply deleting a browser plug-in). The newer stuff makes it as hard as possible for you to get rid of it.

El Capitan has many security changes in it. One at least of which is not allowing such JavaScript loop pages preventing you from closing the page. Click the x to close the tab, and you get a choice to leave the page or stay. You couldn't even attempt to close the tab before, or press the back button.

You can follow an apple article : How to avoid or remove Mac Defender malware in Mac OS X v10.6 or earlier - Apple Support