If the problem did not start until you upgraded Samba, I would tend to think that was the culprit. I understand you reverted to the previous ver. of Samba and used the same smb.conf file, but unless you changed something within XP I would probably focus on Samba as the problem. Unless of course I'm missing something here.
I only recently set up Samba for the first time myself, but I know that the touchiest problem I ran into was getting Windows to authenticate okay with Samba.
Anyway, there's my .02, good luck
BTW, I'm in Maine too woohoo!! (Some winter huh? I mean.. what winter??)