If the setting is defined in a Group Policy, it will be greyed out (regardless of whether you would normally have permission to change it)

To find out what GPO it is set in, you can run Group Policy Results on the computer from the Group Policy Management Console on the server

http://www.microsoft.com/download/en/details.aspx?id=21895

Once you’ve identified the GPO that the setting is defined in, just update the GPO with the new settings and run a gpupdate on the computer to force it to download the new setting