If you can’t connect via mstsc to the workstation and Event Viewer tells you:
Application popup: \SystemRoot\System32\RDPDD.dll failed to load
The problem is with the size of the session image space. The session image space is shared by the display drivers and printer drivers. If the sum of the drivers (rdpdd is an additional display driver) gets too big then there is no room left to load.
If you increase the size of the session image space then you can run remote desktop.
Add the following registry key:
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management]
“SessionImageSize”=dword:00000020
Source: http://social.microsoft.com/Forums/en-US/whssoftware/thread/c41fb638-c422-45e6-8618-1cd37a5e1b81