You can do this via the GPO settings. Enable Remote desktop under:
Computer Configuration > Administritive Templates > Windows Components > Terminal Services
Enable the “Allow users to connect remotly using Terminal Services” setting which will turn RDP on for all machines which the GPO applies. By default anyone who is an administrator on a machine can RDP to it. So to allow domain admins to RDP to machines nothing else needs to be done.
Don’t forget that it can take up to 24 hours for all Windows XP machines to get the updated GPO setting. To decrease this time either reboot all the client machines or run a gpupdate on them from a command prompt.