If you are running ssh (secure shell) on the Zenwalk PCs you can access them from the XP PCs using winscp (a free program). You can also run Samba on the Zenwalk PCs and they will appear to XP to be just another Windows machine with shared directories. The Samba client will allow you to access the XP PCs from Zenwalk.
openssh is available on the Zenwalk website. Winscp is available here: http://winscp.net/eng/index.php
That should get you going. Samba can be a little more involved but it is not too hard. You can get much Samba help on this forum.