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This can take a little effort to fix. A common reason this happens is a trojan, virus or malware made it onto your computer, and one of the things it did was take out Security Center, probably by changing the registry. First thing to do is make sure that your anti-virus software is up to date, then run a complete system scan. Then follow this guide from Microsoft, and hopefully you can get everything back to normal. There is a “fix it for me” tool on this page that you might want to try, but to be honest I have not had much success using it.