Why do fake virus warnings keep appearing in my computer notifications?

Reviewed answer

A website may have permission to send notifications that imitate security warnings.

Do not click the warning. Note which browser or website sent it.
Open that browser's settings and find Notifications or site permissions. Block the unfamiliar site's notification permission.
Check installed extensions and remove unfamiliar ones. If you downloaded or ran something, use the device maker's security guidance.

Get help if

You installed software, entered card details, or warnings continue after the unwanted site's permission is removed.