WARNING WARNING WARNING WARNING WARNING
If you happen to trip across something that triggers a big, screen-filling message that tells you your computer is infected and gives a number for you to call immediately DO NOT DO IT.
Your computer is not infected, and the people on the other end are not legitimate. They will try to convince you to allow them into your computer via remote to "fix" the infections, and they are really threatening. They might tell you they are a "Microsoft Partner". They might tell you there are a lot of infections on your computer and also say if you don't let them in they'll have to disconnect your computer from the Internet (they can't).
Please don't call the number on your screen - it will start you down a rabbit hole you don't want to enter. These people are convincing, relentless and they will keep calling you. If you've already spoken with someone like this and they have your credit card info, cancel the card and get a new one.
You may not be able to close the message or shut down the computer normally. Simply hold the power button in for 5-15 seconds and your computer will go off. Wait a minute, then turn it on and immediately do a clean-up of your computer. In order: run CCleaner, AdwCleaner, Malwarebytes and SuperAntispyware. If the tools don't show any infections, carry on. If they find things that need to be removed, do so.
If you want help with any of this, contact me. You can call, text, email or FB message me ANYTIME - day or night. If you're EVER not sure about a message on your machine, contact me before proceeding. I don't charge to answer questions!
Cathy Contant firstname.lastname@example.org 315-573-4905