Dear @J Rivers
The messages you are seeing, such as “Trojan Virus Found,” “Critical Error,” or “System Error,” are often fake security alerts generated by malicious websites or browser notifications.
First, DO NOT CLICK anything in the warning and do NOT CALL any phone number shown in it. Do not download software recommended by the message or provide your Microsoft password, MFA codes, payment information, or remote access to anyone. If the warning is currently displayed in your browser, close the browser. If it will not close normally, press Ctrl + Shift + Esc, select the browser in Task Manager, and choose End task.
If you are using Microsoft Edge, go to Settings --› Cookies and site permissions --› Notifications and review the sites listed under Allow. Remove or block anything you do not recognize.
I would also check your browser extensions. In Edge, go to Settings --› Extensions --› Manage extensions and remove anything unfamiliar or recently installed.
After that, run a full Defender scan from Windows Security --› Virus & threat protection --› Scan options --› Full scan. If you still receive the warnings, run Microsoft Defender Offline scan from the same location. This will restart the PC and scan the system before Windows fully loads. You can also review what Microsoft Defender has detected by going to Windows Security --› Virus & threat protection --› Protection history. This page shows recent threats and potentially unwanted applications detected by Microsoft Defender, including whether they were blocked, quarantined, or require further action. If you see an item you do not recognize, I would recommend checking its details before taking any further action. https://support.microsoft.com/en-US/Windows/Security/windows-security/protection-history-in-the-windows-security-app
It is also worth checking Settings --› Apps --› Installed apps and sorting the list by installation date. Look for applications you do not recognize or did not intentionally install.
If you have already clicked the warning, entered your credentials, downloaded anything from it, or allowed someone to remotely access the PC, let us know. In that situation, I would recommend changing the affected credentials from a trusted device and reviewing the account's recent sign-in activity before continuing to use the affected computer for sensitive activities.
I hope this help.
Best regards,
Bence