Here's a great tip from Frank Rojas, a real life Support Engineer in North Carolina. If you having trouble with Remote Desktop on some of your clients and can't figure out why then this may be your issue:
Issue: Remote control is setup in ConfigMgr 2007 via the Remote Tools Client Agent. Remote Tools, Remote Assistance, and Remote Desktop work fine on Windows XP clients, however, only Remote Tools works on Windows Vista clients.
Note: Remote Assistance to Windows Vista clients is only supported from ConfigMgr 2007 Admin consoles being run from Windows Vista or Windows Server 2008 PCs. Remote Assistance to Windows Vista clients will not work and the option will not be available in ConfigMgr 2007 Admin console being run from Windows XP or Windows Server 2003 PC.
Cause: This problem is caused by Network Level Authentication being turned on under the Remote Desktop tab of the Remote Tools Client Agent. If the Operating System of the PC trying to initiate the remote control session does not support Network Level Authentication it will fail to connect. Currently, the only Operating Systems that support Network Level Authentication are Windows XP SP3, Windows Vista, and Windows Server 2008.
Resolution: To fix the problem, take one of the two following actions:
For information on how to configure the Windows Firewall ports for Configuration Manager clients and remote control see http://technet.microsoft.com/en-us/library/bb694088.aspx.
For more information on Network Level Authentication see Understanding Network Level Authentication at http://technet.microsoft.com/en-us/library/cc505913.aspx.
For more information on Remote Tools in Configuration Manager 2007 see http://technet.microsoft.com/en-us/library/bb681039.aspx.
J.C. Hornbeck | Manageability Knowledge Engineer