This article deals only with remote assistance, not with the normal Remote Desktop.

When you offer remote assistance either through an invitation or directly, you get the error message, "The remote server machine does not exist or is unavailable".

First of all, check the target, i.e. the novice's computer, also called the remote desktop server, for the following settings.

1. Right-click on: My computer
2. Click on: Properties
3. Click on the tab: Remote
4. Make sure the "Allow remote assistance invitations ..." option is selected.
5. Click on the button: [Advanced ...]
6. Make sure the "Allow this computer to be controlled remotely" option is selected.
7. Make sure that the time settings are sufficient. If you are not concerned about long time spans, set it to 30 days.
8. To save these settings and close the dialog boxes, click twice on the button: [OK]

Two different procedures for remote assistance exist:

1. Novice invites assistance by expert. Expert connects, using the invitation.
2. Expert offers assistance by actively connecting to the novice's computer without using an invitation. This requires special settings.

All Windows XP computers require several special configuration settings to allow accepting remote assistance offers, i.e. an expert actively connecting to a novice without prior invitation. They can, however, invite remote assistance without these special settings, i.e. a novice sends an invitation to an expert, and the expert uses the invitation (file or messenger action) to connect. Thus, if in doubt, let the novice send an invitation, because it is easier.

The missing settings for offering remote assistance are described in detail in the article: Windows XP SP2 Firewall blocks offers of Remote Assistance

If one or both computers are connected to the Internet through a router (including NAT), the question is whether the router has UPnP (Universal Plug-n-Play). With UPnP, an invitation by the novice should work automatically. For the different connection scenarios read: Supported connection scenarios for Remote Assistance

If the target computer is behind a router (including NAT), but the router does not have UPnP, read: How do I... Provide remote assistance on computers behind a NAT device?

If it still doesn't work in spite of these settings all being correct, check for the following registry key and rename (or delete) it, if it exists:

HKEY_LOCAL_MACHINE
 \System
  \CurrentControlSet
   \Enum
    \Root
     \LEGACY_RDSESSMGR

After you have done that, enter the following command in a command line window or into the Start, Run ... command to start the session manager:

sessmgr -service

Then try again to establish the remote assistance connection.

Additional information—configuring the client and server for a different port:

Configuring the Remote Desktop Client to Connect to a Specific Port

How to Change the Listening Port for Remote Desktop