Create a file, for example EnableRemoteDesktop.reg, with the following text:

EnableRemoteDesktop.reg

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.