Screen picture of a fake anti-virus pop-up appearing on the screen of my Macintosh, running OS X - but it appears to be a Windows XP desktop and Security Center message.  I opened the Mac OS system info window to highlight this is NOT a valid security warning - but on a Windows XP computer, it might appear so. Note the poor grammar (prevent your PC from crash) in the "Virus Doctor" window, trying to get me to avoid the default action when I tried to close the pop-up window.  If I were running Windows in this case, if I clicked anywhere in the "Warning, threat detected" window, I might be downloading whatever malware the page designer intended, or taken to a page to purchase their "anti-virus" software via credit card.