A pop-up says your computer is infected

The screen suddenly goes full-size, an alarm sounds, and a message says your computer is infected and you must ring a number immediately. It is designed to frighten you.

In almost every case, nothing is wrong with the computer. It's a web page pretending to be a warning. The danger only starts if you ring the number.

How to tell it's fake

It appears while you're browsing a website, not from your antivirus program
It gives a phone number to call — real security software never does
It uses sound, flashing or a countdown to rush you
It claims to be from Microsoft, Apple or Windows Defender

How to close it

Don't click anything inside the warning, including buttons that say "close" or "cancel".

Press and hold the Esc key — this often exits full screen
On Windows, press Ctrl, Shift and Esc together to open Task Manager, then end the web browser
On a Mac, press Cmd, Option and Esc together and choose Force Quit for the browser
If all else fails, hold the power button until the computer switches off
When you reopen the browser, don't restore the previous pages

If you rang the number

The person who answers will try to get onto your computer and then ask for payment. If that's already happened, follow the steps in our guide to fake tech support calls: disconnect from the internet, ring your bank if any card details were given, and have the computer checked.