I have had the popup that says you have been infected do you want to scan,
and if you click on any part of this popup it will install a trojan on your computer that is almost impossible to escape. Unless you can do a system restore to a previous date, you will have to reformat your harddrive and reinstall your OS. I was infected with one of these off facebook about 6 months ago and it took me over a week to recover. I recently went to a site and as soon as the site came up I got that popup. I promptly did a "CrtL-alt-del" and stopped all applications then I rebooted. I wasn't taking any chances. They make these popups look just like the a real warning.