A page says my computer is infected.
Do not call the number on the page. Do not click anything on it. Real warnings from Microsoft or your antivirus never give you a phone number to ring.
Close the page. If it will not close, turn off the Wi-Fi or unplug the network cable, then call us. Shield shows a warning box over pages like this, but only you can decide what to do next. Calling early costs you nothing.