My problem is that windows 7 logs onto both of my ubuntu accounts automatically. I've tried to configure things to require user authentication. I have ubuntu 9.10. Here are the mostly relevant bits of etc/samba/smb.conf. I've followed your win7 instructions here.

#======================= Global Settings =======================


   workgroup = usnthem
   server string = %h server (Samba, Ubuntu)
   dns proxy = no

####### Authentication #######
   security = user
   encrypt passwords = true
   passdb backend = tdbsam
   obey pam restrictions = yes
   unix password sync = yes
   passwd program = /usr/bin/passwd %u
   passwd chat = *Enter\snew\s*\spassword:* %n\n *Retype\snew\s*\spassword:* %n\n *password\supdated\ssuccessfully* .
   pam password change = yes
   map to guest = bad user

#======================= Share Definitions =======================

   comment = Home Directories
   browseable = yes
   read only = no

#  This seems to help
   path = /home
   valid users = %S

This might also be relevant:

$ sudo pdbedit -L -w
mary:1001:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:159EA51341BD0FB531D630F25B2672BD:[U          ]:LCT-4C114AC6:
mark:1000:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:32C5FA7B2B28B47B6959536DC914C4D0:[U          ]:LCT-4C11755A:
Win 7 just gives instant access to the complete directory trees of mark and mary without presenting a logon dialog. What am I missing?