This is acaully a simple problem (Though WHY on earth would someone acually WANT secrurity center when all it does is whine when you dont have what it wants you to have, and you can manually download updates lol ;a)). And im assuming you cant find it when it starts up anywhere and it wont warn you if you dont have something it wants you to run running.
The firstest thing to do is first disable your AV and see if Security freaks, if it does then you have nothing to worry yourself about, if it doesnt follow these instructions!
Dont panic lol
Go to Start - Control Panel - Admin Tools - Services
Find Security Center and Double Click on it
Make sure there is a Path to Executable if not it should be Root:\WINDOWS\System32\svchost.exe -k netsvcs
Startup type should be enabled as Automatic, and then click start in the service status. Click Apply and it SHOULD work. Restart if it doesnt apply right then.
If it doesnt show on Services, see if you can download the file. ;a)