Can I Remove A Personal Antivirus?

I recently got something on my computer that is really quite annoying.  Every time I turn on my computer, I begin to get messages telling me that my computer is at risk and prompting me to purchase a license for an antivirus software.  Apparently, what I have is a personal antivirus that must have slipped into my computer at some point while surfing the Internet.  Does anyone know how to remove personal antivirus?  I am tired of getting false error messages and having notices pop up that try to con me into buying something that I don’t need?  I’m tired of it.