I was not sure about the rules on links to other sites, even if it was free....I will check the rules section and provide a few....

If you have the option of doing a refresh or a clean start as suggested it is never a bad idea.... I recently had a PC with the same issue, upon further investigation I discovered a ransomware text file floating around. While the infection had not completely taken hold yet, it had started to take some permissions, like starting the security center, disabling my on-access virus scanner and disabling system restore....

I was able to recover my antivirus by removing and reinstalling (permissions were different on the reinstall)... Scanned and located enough infections to give me some clues to dig around and kill processes, prevent the restart and finally take permissions back on my services.....

There are some nasty variants of ransomware/malware going around.... There are a few free cleaners and root kits around that help but spotting them early is key.... For example while it may not be a big deal that your security center  will not start, but that in itself may just be an indicator of a much larger issue....

Good luck.
S

You need to provide more information.
What operatingsystem are you using and version?

If it is Windows than you can start with a Clean boot in a minimal configuration and then run the mrt.exe to get a first scan of your computer.
To make a cleen boot.
http://www.thewindowsclub.com/what-is-clean-boot-state-in-windows