Is this a scam?

I don't have Norton Security installed on my computer. Sometimes I'd get these very sudden, very abrupt pop-up of Norton Security telling me I have a virus and doing an actual virus scan. Then another pop-up appears saying I have to upgrade Norton Security. Now I thoroughly check to confirm if I have any viruses, and everything comes up positive. So why does this pop-up of Norton Security happen? I don't have it installed, so why does it randomly appear as if I did have it installed?