Security problem with an ad by supdotcom "11 wedding photos that went horribly wrong"
This ad, when clicked on caused popups claiming a virus had been found and my computer screen locked until i called a given phone number. When the tab that opened on the click, labeled "security" was closed the popups disappeared. A malware check revealed no virus. This ad seems to be a scam to get people to pay to have their computer "fixed".
Mike Enerson commented
I had something like this happen too. Since I have gotten no response, I can only assume that Yahoo does not care that they are spreading viruses or worms.