Unexpected Pop-up Ads from System on my Windows PC

Your pop-ups occur without a browser despite scans, so a background app, scheduled task, or potentially unwanted application is more likely. Do not click an alert; capture wording and time. Save your work, then open Windows Security > Virus & threat protection > Scan options and run Microsoft Defender Offline, which restarts the PC and scans before Windows loads. Under App & browser control > Reputation-based protection settings, enable potentially unwanted app blocking for apps and downloads. If alerts continue, perform a clean boot: in msconfig, hide Microsoft services and disable the remainder; disable startup apps in Task Manager and restart. If the ads stop, re-enable items in groups to isolate the source. Microsoft Sysinternals Autoruns can then show third-party logon entries and scheduled tasks; uncheck a suspect entry instead of deleting it. The source cannot be confirmed without the screenshot or identified entry, so preserve that evidence before removal.