windows security center does not find antivirus software

After using nis2011 and vista premium  for many months the windows security center icon pops up in the system tray after a reboot and reports that "windows did not find  antivirus software on this computer."    My nis2011 is up to date and shows that all protection is on. 

 

Why this message? 

 

Sorry for the first post by mistake on Norton 360.