This policy setting specifies whether Security Center is turned on or off for computers that are joined to an Active Directory ...

This policy setting specifies whether Security Center is turned on or off for computers that are joined to an Active Directory domain. When Security Center is turned on, it monitors essential security settings and notifies the user when the computer might be at risk. The Security Center Control Panel category view also contains a status section, where the user can get recommendations to help increase the computer's security.  When Security Center is not enabled on the domain, neither the notifications nor the Security Center status section are displayed. 

Note that Security Center can only be turned off for computers that are joined to a Windows domain. When a computer is not joined to a Windows domain, the policy setting will have no effect.

If this setting is Not Configured, the Security Center is turned off for domain members. 

When this setting is Enabled, Security Center is turned on for all users. 

When this setting is Disabled, Security Center is turned off for domain members.

Windows XP SP2
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In Windows XP SP2, the essential security settings that are monitored by Security Center include firewall, antivirus, and Automatic Updates.  Note that Security Center might not be available following a change to this setting until after the computer is restarted for Windows XP SP2 computers. 

Windows Vista
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In Windows Vista this setting monitors essential security settings to include firewall, antivirus, antispyware, Internet security settings, User Account Control, and Automatic Updates.  Windows Vista computers don't require a reboot for this setting to take effect.