Non-RDP Remote Desktop Solution for LAN – Windows Home Premium

Actually you can do this from the computer you are trying to connect. They've changed it in vista so that you can't anymore access shadow console directly. You must first log in to remote computer (using other account) and then shadow the console.

See more detailed instructions here
http://thegreenbutton.com/forums/p/52888/238449.aspx

If someone knows a way to connect directly to current users session without locking him out, or even letting him know, I would be also interested.

The error message seems to imply that the port is not available to those systems.

Maybe there is a layer 2 firewall filtering the connections (or a firewall on the server). Check this by using telnet (from the client, open a command window and type telnet <serverip> <rdpport>). If it does not respond with a banner announcing the RDP service, the problem is at the network - i.e. a firewall of some sort) rather than the app.

If your network is more complex, it's also possible that while the computers are on the same LAN, they are on different subnets, and there is a regular firewall blocking connections between the subnets. You may be able to check this using traceroute. If there is more than 1 hop, or the first hop is not the IP of the remote system, then you are going through a router - most likely with a firewall preventing access.