Folder sharing problem

Scenerio:
I have an XP-Pro and an ME machines in a home network connected to a
broadband router to a DSL modem. The XP machine has two folders setup for
R/W sharing for the ME machine. The ME machine would map the folders into
drives and can open/save files. However things start acting strange since
yesterday that some subfolders cannot be browsed and today some files
cannot be saved. I checked the XP shared folders and found that one top
folder had READONLY attribute set. I unchecked it and apply to all
subfolders and files. But then it reverted back to READONLY. The other top
level folder works OK.

Help Appreciated:
What caused the folder to become READONLY ?
How can I fix this problem folder ?

Thanks for your input !

Patrick