This is critical, and if you don't you will get hacked into next week!!
Use netbeui for netbios on your lan, (ON XP it's on the cd in the valueadd folder), or disable netbios over TCP/IP on your internet connected nic and leave it enabled on your lan nic.
To do this:
Rioght click your internet conected conection, goto advanced TCP/IP properties, then click the wins tab.
Click the box "Disable netbios over TCP/IP.
Nothing over the internet has any reason to have netbios access over TCP/IP so nothing will break on you.
Cheers!