The "Windows Defender Security Center" popup message below is a scam. The scam, which claims the potential victims' PC has been blocked for security reasons due to spyware or malware threat, is being used by cybercriminals or scammers to frighten and trick potential victims into calling them at 1-865-484-6972, a fake Windows Support telephone number. Calling the fake support telephone number will connect potential victims to the scammers, who will pretend to be from Microsoft. They will attempt to trick their potential victims into disclosing their account credentials, personal and financial information which they will use fraudulently.
The Windows Defender Security Center Scam
Potential victims will also be asked to give the scammers remote access to their computers, something Microsoft will never ask their users to do.