Isabel,
From the error you are getting, a possible explanation is that you are using the wrong IP address to connect from the first computer to the second. Please do the following for verifying that.
1. On the first computer, run a cmd console (click the Start button Picture of the Start button. In the Search box, type cmd, and then, in the list of results, double-click on the cmd application).
2. In the cmd console run the following command:
> ipconfig
3. Do the same on the second computer.
4. Copy&paste the output you get on the first and second computer and send us via mail at: forum[at]nomachine[dot]com. For privacy, it’s better you don’t post the IP addresses in a public forum