Tip - A Quick Tip To Enable Terminal Services Access For Previous Versions of Windows
Allow previous versions of Windows to access Windows Server 2008 Server Core using Terminal Services.
Windows Server 2008 runs in higher security level. By default, Windows Server 2008 does not allow previous versions of Windows computers to access the Server Core via Terminal Services. You must turn off the higher security level to allow them to access via Terminal Services
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http://www.windowsnetworking.com/kbase/WindowsTips/WindowsServer2008/AdminTips/TerminalServersAndThinClients/AQuickTipToEnableTerminalServicesAccessForPreviousVersionsofWindows.html