It could be I'm unique when it comes to my setup but I doubt it. The XP and Windows 7 machines have no passwords set and use simple file sharing. 

In order to see the folders in Windows 7, the folders need to be shared. In the process of sharing, the file permissions are automatically changed so that they will share properly.  This works fine until you copy something into a shared area - that file or folder is not network accessible, you can see it in 7 and change into the folder but you can't open or copy the folder nor the files inside it.

What you need to do is remove the share on the folder and then re-apply the share.