No antivirus in the world will give you 100% protection. You have to be careful what you download, click on, share etc.
Read this article on how to remove this fake anti-malware: http://www.bleepingcomputer.com/virus-removal/remove-antimalware-doctor
Scroll down that page, do NOT click the first links you see as they are adverts. That is the best removal guide on the web and is easily found using a Google search
Make sure that Windows is totally up to date with both critical and non-critical updates as that can leave you open to infection, especially if your Internet Explorer is not up to date and at its default settings (even if you don't use it).
I moved this to the Malware removal section of the board by the way.