You must first be logged 
      on as an administrator or a member of the Administrators group to enable 
      Remote Desktop on your Windows XP Professional-based computer.
      
      1. Either right My Computer icon on desktop or open the System folder in Control Panel. Click Start, point to 
      Settings, click Control Panel, and then double-click the System icon.
      
      2. Click the Remote tab, and tick both boxes "Allow Remote Assistance 
      invitations to be sent from this computer", and "Allow users to connect remotely to this 
      computer" check box, as shown below. Then Click OK to clear the warning, 
      click ok.
      
      
      3. To invite a friend to 
      connect to your computer, Click Start, Help and Support, then under 
      heading "Ask for assistance", Click "Invite a friend to connect to your 
      computer with Remote Assistance" and follow instructions.
      Note If you're running 
      Windows XP Service Pack 2 (SP2) and you enable Remote Desktop, Windows 
      Firewall will be automatically configured to allow Remote Desktop 
      connections to your computer. However, Remote Desktop will not work if you 
      have Windows Firewall configured to allow no exceptions. To allow 
      exceptions in Windows Firewall, in the control panel open the Security 
      Centre, click Windows Firewall and clear the check box next to Don't allow 
      exceptions. Typically, remote desktop is set to use port 3389, if you have 
      an external NAT enabled router or external firewall, you will need to 
      allow incoming port 3389. You will need to setup port mapping on your 
      router firewall if you are using NAT.