Connecting troubles with Win XP and NAV 5.0 SP1

Hello people

I've installed the Navision Server 5.0 SP1 on a Windows XP Client. The Customer doesn't have a real server, but this client is used as server. Anyway, the installation was easy. Then I've installed a NAV Client to check the connection. After modify the Hosts file, the connection was working trough the server (NAV Service).

But I can not login to the NAV DB from a other PC. I can ping the service by service name, without problems. But NAV gives me an error back (the time is up......). What can I do? Are there any permissions to set? The customer use a very simple network (TCP).

Thanks for your help.

regards from zurich, curdin