suddenly all users except markp could not access users within the ‘shares’ (shared) folder.

Tools to test:

see log on the server at /var/log/samba/log.smbd

see if the access problem exists on the server, as well as the windows client;

smbclient //localhost/share_to_test -U annet

problem in my instance (Feb 2011) was that the group name of the /home/shares folder had changed from ‘john’ to ‘users’
Probably caused by Unison sync foulup at my end (user 1000 at home overwrote user 100 at TC).