If a pop-up is asking you to do something immediately, it’s probably a scam. You should also be suspicious of any message that tells you to call a phone number. Companies like Windows and Google have other means of contacting you, and they’ll always use more official channels. Pop-ups loaded with flashing lights and spelling mistakes are also highly suspect.
Use your best judgment, but I also recommend running an antivirus like Norton, which will warn you about scams and malware.