Is my computer really infected when the box says so?

No. It is an ordinary web page built to look like a warning, complete with an alarm sound and a logo. It cannot see anything at all about your computer. Once you close the tab, everything is as it was.

Can Microsoft or Apple call me about my computer?

No. Microsoft, Apple and Swedish banks never call you about your computer and never show phone numbers in a pop-up. They also never ask you to install remote-control software or to read out a code. A call like that is always an attempt at fraud, whatever the number displays.

I already called the number, what now?

Disconnect the computer from the internet, restart it and uninstall the remote-control program they had you install. Then call your bank and block cards and BankID if you paid or identified yourself. Change your email and banking passwords from another device, and report it to the police on 114 14.

Fake virus warning filling your screen? How to close it

A box suddenly fills the screen saying your computer has been locked by Microsoft or Apple, that it is infected, and that you must call a phone number. There is often an alarm sound, and the page will not close in the usual way.

This is always an attempt at fraud. Your computer is fine. It is an ordinary web page built to look frightening, and it cannot see anything about your machine. Microsoft, Apple and Swedish banks never put phone numbers in boxes on web pages. Do not call the number, just close the page.

Try this first

Do not call the number. Whoever answers wants to take remote control of your computer, charge you for something you do not need, or reach your bank. The number may well start with a Swedish area code.
Leave full screen. Pressing Esc, or F11 on Windows, brings the tabs and buttons back so the page can be closed.
Close the tab with its cross. If that does not work, close the whole browser.
Force it closed if the page is stuck. On Windows: hold Ctrl and Shift and press Esc, select the browser in the list and click End task. On a Mac: hold Command and Option and press Esc, select the browser and click Force Quit.
Open the browser again. If it offers to restore your tabs, say no. Otherwise the same page opens straight back up.
If the sound is still playing, restarting the computer takes care of it.

If that did not help

Clear the cache and cookies in the browser so the saved copy of the page goes away.
Look through your extensions. In Chrome: the three dots, Extensions, Manage extensions. Remove anything you did not install yourself.
Check the start page. If the fake page opens every time you start the browser, go to Settings, On startup, and remove the address listed there.
Scan the computer. On Windows: type Windows Security in the search box next to Start, open it, choose Virus and threat protection and click Quick scan. It is built into Windows and does not need buying. On a Mac no scan is needed for this.
Switch off website notifications. If boxes appear even when the browser is closed, they are notifications you approved, and they are switched off under Settings, Privacy and security, Site settings, Notifications.
Download nothing the box recommended, and never pay for a cleaner program offered in an advert.

If you already made the call

If you let someone into the computer through a remote-control program, usually AnyDesk, TeamViewer, UltraViewer or Supremo, do this in order:

Disconnect the computer from the internet. Unplug the network cable or switch off Wi-Fi.
Restart the computer and uninstall the remote-control program under Settings, Apps, Installed apps, or drag it to the bin on a Mac.
Call your bank from your phone and block cards and accounts if you paid, identified yourself with BankID or gave out card details. Banks keep a blocking line open around the clock.
Change your passwords for email and banking, but do it from another device such as your phone. Always type passwords and codes in yourself and never read them out to anyone, including us.
Report it to the police on 114 14.
Have someone go through the computer before you use it for banking again.

If you are unsure whether a call was genuine, hang up and call back on the number printed on the bank's own website or on the back of your card. A real member of staff has no objection to you doing that.