ok, to see if the contents are hidden, choose the folder in the finder and press command+i to list the folder info. Look to see if the "locked" checkbox is selected.
To delete the .ds files (perfectly safe by the way) download onyx from versiontracker.com, run the program and choose the appearance tab, check "show hidden files and folders" and check the "prevent .DS_store creation over network".. Then under the maintenance tab check "reset display of folders contents" and click execute to run all 3 items. After a reboot open the folder you're trying to view and delete the .DS files. Then try to see the folder on the windows machine again.
Also check the firewall on both XP and the Mac for file sharing ports, it sounds like this is not the issue though as the folder does show up in XP, it may be a permissions issue.
Better to use a Mac and be THOUGHT a fool, than to use Windows and REMOVE ALL DOUBT