Run the fake alert stinger. Note to get it updated you need to uninstall it and get a new copy as it updates daily
Moderator ex_brit posted this
Unfortunately antiviruses generally aren't very good at these fake anti-malware pests but there are tools available, free, that will kill it.
If necessary boot into Safe Mode with Networking by tapping F8 repeatedly while booting up if your desktop wont download or run anything.
Also you could try System Restore to before this happened, in which case, if successful, you should then temporarily disable System Restore to get rid of the infected restore point.
There's an excellent removal guide here: http://www.bleepingcomputer.com/virus-removal/remove-security-defender
+++Scroll down that page, do NOT click the first links you see as they are all advertising which helps pay for that web site.
If you go the restore route don't forget to update everything afterwards.