Stop clicking on anything and don’t enter any passwords. Disconnect from the internet if you can (unplug the network cable or turn off WiFi), then run a scan using your antivirus software — on most Windows PCs, that’s the built-in Windows Security app. If the scan finds something serious, or you’re not sure whether it’s actually gone, stop there and call us rather than continuing to work on the machine.
Watch/read: Microsoft’s guide to scanning for threats with Windows Security
