The Windows Security Center Shield in Windows Vista
Windows Security Center is a program that monitors firewalls, antivirus, and Windows Update. The Security Shields such as X for Red, ✓ for Green, ? for Blue and ! for Yellow.
History
Windows Security Center first appeared in Windows XP SP2. This was one of the major improvements from Windows XP SP1 and earlier Windows versions. It was made partially because of the better Windows Firewall from the obsolete Internet Connection Firewall.
Windows Vista
Windows Vista improved it by making it more noticeable in the Control Panel.
It also splits Antivirus into Antivirus Software and Anti-Spyware and other anti malware.
Windows Vista's Control Panel makes it much more easier to find Windows Security Center.
Windows 7
Windows 7 build 6801 and 6933 has an improved WSC.
Windows 8 and 8.1
Windows 8 and 8.1 replaced the antivirus programs (etc.) with guest/public and private networks.