Beware the Fake Antivirus Alert
So you're surfing the Internet and suddenly a window pops up claiming your computer's infected with a virus. The pop up may look like it's from your installed antivirus software, or it may just be a plain vanilla warning.
If the window provides a telephone number for you to call, DON'T CALL IT. Regardless of which antivirus you might be using, (Norton, McAfee, Kaspersky, Windows Defender, Eset NOD32, et al) none of them will ask you to call someone if the software discovers a virus. If you call the number you'll talk to a "technician" (usually not in the US) who will claim to remove the virus if you pay a nominal fee by credit card. Your card number will promptly be compromised, making the "infection" far more costly than the fee you authorized. Then they may ask to remote into your computer, promising to "fix" the problem. DON'T LET THEM. They won't be removing a virus, they'll be planting at least one, making your computing life miserable.
So what should you do? Close all the open windows, and run a scan using your antivirus software. However, don't be alarmed if you don't find anything. If they haven't installed anything remotely, a reboot of your computer may be your best defense (even if the pop up advises against it). In addition to your antivirus, you may also want to run an anti-malware software, such as Malwarebytes, which you can get free from Malwarebytes.org.
How did this happen? The bad guys purchased space on legitimate website, and inserted a seemingly innocent ad which, when you get to that page, automatically runs a script (a kind of software) that produces the pop up window. All this is in the hope that you'll panic and call them. Don't fall for it!