How come every so often, as in reasonably often does a little yellow speech bubble appears in the bottom left hand corner of the screen telling me that I 'may not have virus software or a firewall installed' it then directs me to the windows security center.

I do have a virus and firewall, its Norton Internet Security and it is fully installed with a virus protector and firewall. So what I have done is clicked on the option which tells the computer that I have software installed and I am monitoring it myself, yet the yellow bubble still appears.

Why is this? It’s really annoying.

You can stop it notifying you by going to the control panel then windows security center. Under resources select the "change the way security center alerts me" tab. Uncheck the appropriate boxes.