Access denied even though I have full control

This is a wierd problem, and one so generic I can't even figure out
where to post it.

My home setup has a Primary Domain Control and file server running
Windows 2003 R2, and a Desktop running XP SP3, and a laptop.

I have some folders on the PDC shared to the network.

Suddenly, I started getting "Access Denied" anytime I try to write to
a folder on the shared drive from the desktop. This is true even
though I have "Full Control" and even ownership of the shared
directory. I can, however, write to those folders from the Laptop,
when logged on as the same user.

I can't figure out what made it start happening and what to do to fix
it. I installed ZoneAlarm Security suite at about that time, but I
have disabled it (as in, making sure it doesn't even start), and I
still have the same problem, so I really don't think that's it.

Any hints at where to look. I'm not a neophyte (though I'm not a
System Administrator), but I've tried everything I can think of.