Fake virus messages have been popping u i browsers for years. A pop-up window alerts you that your computer is infected with viruses and the only option to get rid of the pop-up is to click OK. It encourages you to scan your device with a specific antivirus software program and then offers to clean the bogus bugs off for a small fee. If you download the program, it installs malicious software on your device, and then the scammers have your credit card information.
Do not click on any links in pop-up ads. You can dismiss the ad without rebooting your whole computer — just forcibly quit the browser program. Install a robust, trusted antivirus program on your computer and run it regularly. Also install pop-up blockers on your browsers to prevent these scams from ever reaching you. Keep the important files on your computer backed up to an external drive, just in case.