The way I understand it, Netbios runs off of UDP and the network neighborhood uses this when accessing other machines. When you open network neighborhood on a peer to peer network your machine broadcasts a UDP signal to other machines looking for themt to find a "master browser" with the location of the other machines. The signals that you see on your firewall from the other machines on your network are probably just the machines looking for each other on the network. My 2000 machines on my network "talk" to each other with netbios every few minutes. Some machines talk more than others but I have noticed that the "chatty" machines are the ones I use the most and have the most services running.

Now, if you get a netbios signal from outside of your network - you may have a problem.