support on LAN issue requested

History & setup
I have just switched from Windows to Ubuntu.
I have a LAN under Windows consisting of 4 computers.
Just 2 of them have been switched to Ubuntu, my desktop and my laptop.

Network works fine when it comes to connecting to the internet via hubs & WIMAX.
I can access my LAN printer without problems.
I can find my other computers logged in on Windows and access files on them.
But I simply cannot connect to my laptop from the desktop or vice versa when logged in on Ubuntu.
I tried everything that I could find under 'Help', followed setup as described for 'shares-admin' etc.

rgds
Wolfgang