This has come up several times recently — complaints like: “I get so many ‘Popup Ads’ I can’t use my computer anymore..”
Visit this link: http://support.apple.com/en-us/HT6506
Ad-injection software is advertising-supported software that can come from third-party download sites. Software you download from such sites may have been customized to install both the software you want and the ad-injection software. If your Mac has ad-injection software installed, you might see pop-up windows, ads, and graphics while surfing the web, even if “Block pop-up windows” is selected in Safari preferences. Ad-injection software might also change your homepage and preferred search engine.
Have you downloaded software from third-party providers lately?
Be careful out there! If you have questions, it’s always best to phone or email us first. Our policy is to answer basic questions for free. “Snappy answers to silly questions” (remember the MAD Comic?) are our specialty, and there is no charge. Why not ask first instead of suffering the consequences later?