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Asyn,

Thank you for the info. It seems like the Malwarebytes Anti-Malware recommended in the link you included works much better than Windows’ malicious software removal tool https://www.microsoft.com/security/pc-security/malware-removal.aspx.
Malwarebytes detected over 150 malicious items (while Windows’ removal tool found 0), and upon removal of those items the pop up seems to have stopped.
I will keep you updated if it becomes a problem again.
Thank you again.