What? Macs aren't immune to this. What am I talking about?
Well, this hijack told them that Microsoft had detected a virus. They thought that was a bit odd, but they did go as far as calling the number. Their suspicion was too high for them to give a credit card, though, so they called me.
These things are usually easy to fix. You might even be able to Command-W to a new window. If not, you can Force-Quit. Start Safari again with the Shift key held down and then fix the Home Page. If necessary, throw away Preferences or revert to a Time Machine backup of your ~/Library.
There have also been numerous instances of telephone calls from people who have "detected a virus on your Windows server". If I have the time, I like to string those people along for as long as I can. It wastes their time; time that might otherwise be spent stealing people's money.