Read Only attribute unremovable ??

Desktop and laptop both on XP Pro SP3.

I have Cad and other software on a laptop, and I sometimes use a desktop as
a file
server. Common peer networking situation, never had a problem before. I've
used software on one machine open files on the other (both directions),
hundreds of times over several years.

On the desktop, did some cleanout of old files and backing up stuff
(ordinary copying) to an external drive
(done this dozens of times on this machine). Now I have a problem with the
Read Only attribute on folders. Never set it before, doesn't appear on
individual files, just folders.
When I try to open desktop files from the laptop, I get a message telling me
"The file is not available for write access. Open read only?"
When I go to the desktop and look at Share properties on the highest level
folder, "Allow network users to change..." is checked.
If I Rt-click-Properties from Win Explorer on that same folder, the Read
Only box is checked/colored in. It will allow the box to be cleared, and
when Apply is clicked, a Confirm window comes up. "Apply Changes to
subfolders, files, ..." is checked. When I click OK, the Applying
Attributes process runs through bunches of subfolder names, etc for about 30
seconds and the Read Only box is clear. I then OK out of the folder
properties.

At this point in win explorer, I can immediately go right back to properties
on the top level folder, and Read Only shows as selected again, and the
original problem is still evident.

I was thinking it might be something with the Cad software, but I just tried
an Excel file and had the same problem. Also tried turning off the firewall
on the desktop, no help.

Any ideas on how the Read Only is being set, and how to get rid of it?