Create a file, for example EnableRemoteDesktop.reg, with the following text:
Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Terminal Server] "fDenyTSConnections"=dword:00000000
Then import it with regedit:
REGEDIT /S EnableRemoteDesktop.reg
After which you need to restart the server to have it take effect:
SHUTDOWN -r -t 01
You should now be able to login to the server using rDesktop.