security center

The norton antivirus is installed in my computer and in the security center it was showing that windows is able to recognise the antivirus but after few days, one day I found that security center is notifying me that it cannot find any antivirus installed in my system. After seeing this I installed the norton and then reinstalled it again but it is still showing the same security center at the right bottom corner of the task bar giving warning that antivirus is not installed. So I want to know what is the problem?