put the win 7 disk in the drive, select "upgrade"
This reloads all the default OS files. But does not erase your personal stuff.
I'm not even going to ask HOW the win 7 security was erased. No Comment (lol)
Microsoft Windows 7 Security Center is a feature within Windows 7 itself. This means it is unlikely that it has been uninstalled.
Assuming you are having trouble locating the Security Center within Control Panel, you may actually be experiencing corruption of your operating system as a result of a malicious application. It’s always wise to run a virus scan periodically to confirm the clarity of your machine.
To restore full functionality within Microsoft Windows 7 you may want to run a Windows 7 repair by reinstalling the system via an upgrade install. To learn more about this process, click here.
To learn more about the Microsoft Windows 7 Security Center, you may find this resource useful.