It could be a bad definition update. It could just be that the service didnt start up right. It could be any 1 of 1000 things. If you are that worried run a scan with Spyware Asylum and check your system. If you are still not satisfied uninstall, run ccleaner and reinstall.
It happens to me from time to time as well. I know that it was just a bad boot. Restart and all fixed.