Microsoft Group Policy Preferences provide a way to centrally activate monitor and computer power management features on Windows 7, Vista and XP machines. However, they require either 1) Windows Server 2008 or 2) a Windows 2003 environment being managed via either Windows Server 2008 or Windows Vista using Remote Server Administration Toolset (RSAT).
Keep in mind that Group Policy Preference settings are not enforced. This means that end users can change preference settings, but the changes will be lost on every GP refresh (default 45 min).
For additional details on using Group Policy Preferences to manage Windows XP and Vista Power Management settings, see Sustainable Computing: Power Management Settings for Compliance and a Reduced Footprint, and Managing Power with Group Policy: Part 3 of 3 .
For general information about Group Policy Preferences, see the Microsoft white paper, An Overview of Group Policy Preferences .