oh, Robi, I know, I had one of these antivirus scams recently, I even asked one of my rare questions about it
http://www.theanswerb.../Question1012237.html
They can look very authentic these days and I thought it was my genuine Windows firewall (which I never look at so I don't know what it's supposed to look like).
Best thing to do if you get one is just click it away and run a proper scan for malware and stuff just in case. I have Malwarebytes, which is free and recommended by people like Buenchico. For viruses I have Avast, ditto, which is on all the time and occasionally tells me it's just repelled a virus. I also seem to have some other cleaner-uppers, probably more than I need, but they don't appear to trip over each other.
Storm clouds are brewing here, you can tell it's open garden day in high summer.