Firstly you can edit the Registry of the remote machine by hand:

Navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Terminal Server on the remote machine, and find the key: fDenyTSConnections
If it doesn’t exist create it, then set it’s value to “0″.

Alternatively you can use the following batch file to do the job for you.
Note: You must be logged into your computer with credentials that allow you to remotely edit the Registry of the machine in question, otherwise the batch file will fail.

syntax: EnableRemoteDesktop.bat RemotePC
NB: This batch file must be run under Windows XP

Save the following to a file named EnableRemoteDesktop.bat

@echo off
setlocal
if {%1}=={} goto syntax

:loop
if {%1}=={} goto finish
set remote="\\%1\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Terminal Server"
shift
reg.exe ADD %remote% /v fDenyTSConnections /t REG_DWORD /d 0 /f>nul 2>&1
if NOT %ERRORLEVEL% EQU 0 @echo %remote% NOT found.
goto loop

:syntax
@echo Syntax: RemoteDesktop Computer1 [Computer2 .... Computern]
goto loop

:finish
endlocal