Unsolicited messages on my PC

Since yesterday i regularly get messages pop up in my desktop.

A certain internet service informs me incorrectly that a trojan viruse has been detected on my system. If I want this “virus” removed I have only to click and agree to certain rules and costly licence terms.

I question this “Virus Find” because I have always had Norton antivirus and Firewall software installed on this computer.

My question is how can I block these messages from appearing.

Thank you

Paul King.