I have a server with kubuntu installed and it is being accessed by several windows pc's. Samba is intalled and everything was working perfectly until I moved all the computers to another room. Previously linux server was sharing internet, but now all of them are connected directly. However there were no changes made to the network but now I cannot see the server from any of the windows computers and smb reports an error that it cannot find the network, and that it's probably due to the firewall.
I don't think there's even a firewall installed (the kubuntu installation was not mine, and I've just started to work with it).

how can I check if there's a firewall? and how can I disable it ?
Where do I open port 445 (I've read it in several threads...)

tks 4 yr help

Let's break this down:

Kneejerk response, are they all connected via the same switch/hub? If not, are there multiple switch/hubs involved that can all see each other?

What were the IP addresses of your network before, and after the move? Assuming the machines only had 1 NIC each before, did you add a second NIC to connect to your internal network? Their default gateways are probably pointing to their ISP now, did you add routing for your internal network too?

Can the PCs ping the samba server? If not, have you run any packet tracing to see where packets are going(ethereal/wireshark, traceroute, etc..)?

If you didn't enable a firewall, and you installed the official kubuntu sources, there is not firewall enabled. Google for iptables to read up on the linux firewall platform.