How do you recognize fake antivirus warnings and tech-support scams?
Real antivirus never blasts full-screen browser warnings with phone numbers, countdown timers, or payment demands. Those popups are the scam: close the browser tab (or the whole browser), never call the number, and run a scan with your real, installed security software.
The "your computer is infected" popup is one of the most profitable scams on the internet, and it works by mimicking exactly the product category meant to protect you. The tells are consistent: it appears in your browser (real antivirus alerts come from the app, not a website), it shouts urgency with timers and alarm sounds, it shows a phone number (no legitimate vendor asks you to call from a popup), and it wants remote access or payment in gift cards.
The right response is anticlimactic: close the tab, or if it has hijacked the window, close the browser via Task Manager (Ctrl+Shift+Esc). Do not call, do not click "remove infections", do not install anything it offers. Then, for peace of mind, run a scan with your actual security software; Malwarebytes is excellent at sweeping out the adware that often spawns these pages.
Warn the people around you, because these scams target the less technical: real companies never cold-call about infections, never demand gift cards, and never need remote access to "fix" a home PC out of the blue. One family conversation prevents more damage than any software.