I don't have Norton any more, but it could well have been the culprit. Now using F-Secure again, as the 2007 version is an improvement on the previous one.
I have actually found the solution by Googling, and it involves resetting the Security Center:
1. Go into Control Panel > Adminitrative Tools > Services and locate Windows Management Instrumentation.
2. Right click and turn off WMI.
3. Navigate to C:\WINDOWS\system32\wbem\ and delete
*only* the Repository folder (this hold the security center data, as to what security software you have installed).
4. Go back and turn WMI back on, then reboot, and the Security Center will be reset, and should display the current security products installed.