First of all, huge thanks for the detailed reply Windependent :D
A couple of things I guess I should have mentioned initially. Firstly, the other computers on my network are operated by trusted family members :). Secondly, each computer is individually secured by a firewall (zonealarm for the XPs, Mandrake Firewall (shorewall?) for me). The XP machines passed the tests you linked, my machine kinda did but some ports were closed instead of blocked (stealthed). Not sure how to fix that yet. However, in this configuration SMB does not work. I have to relax all the firewalls to allow it to work, hence my question.
Given your reply, it is clear to me now that communication between the machines aint gonna happen securely in my current network setup, and by relaxing those firewalls to use SMB, well, it seems that was pretty stupid
I'll be heading out ASAP to pick up a router/firewall, you are right, it seems stupid not to pick one of these up when it will increase security and simplify network maintenance, especially now that they are so cheap.
Hopefully your explanation will also help others.
Thanks
Floyd