Mark says that Norton anti virus on Windows 8 allowed a web search toolbar (called SearchUS.com) to get on Internet Explorer and it's been a pain in the neck to get rid of. Leo says that while it did happen, it's not really Microsoft's fault. They control 95% of the market and that makes it a huge target. Since it could have come with something else he installed, Mark may have accidentally installed it. Viruses can't get installed unless the user runs some sort of program to trigger them. Just having data can't really do it. It may be a security flaw in IE 10 that allowed it, though. Microsoft has learned a lot since the early days and has done a tremendous amount to combat malware. When they control that much of the market, they become a very popular target.
Leo suggests starting fresh. He should back up his data, format the drive and reinstall the OS from a known, good source. In fact, Windows 8 has a refresh OS feature that will wipe everything and reinstall at the touch of a button.