I use remote desktop all the time, but after upgrading to SP3 on XP, the machine I was connected to would boot me off after around 2 hours of idle time. On my machine I got the following dialog:

The remote session was ended because the idle timeout was reached. This limit is set by the server administrator or by network policies.

When I connected to the machine again, it had this dialog:

Session has been idle over its time limit. It will be disconnected in 2 minutes. Press any key now to continue session.

The Solution
It took several rounds to find a solution, so here it is for anyone with the same problem.

1. Run (Windows-R or Start menu Run) gpedit.msc

2. Computer Configuration -> Administrative Templates -> Windows Components -> Terminal Services -> Sessions

Originally, all the settings were Not configured Changing the following entry fixed it.

3. Double click or right click and select properties on Sets a time limit for active but idle Terminal Services sessions

4. Select Enabled radio button

5. Within the dialog, set Idle session limit to Never.

6. Hit OK

You may have to reboot for the changes to take effect.