"Roaming Profiles" is one way, (only for XP Pro) and it's not too easy to setup, see http://hr.uoregon.edu/davidrl/samba.html
If you're using XP Home, the choices are even fewer..
coz the logged in computer name holds the share "username
", you will have to share the directory above the actual user's shares and rely on directory permissions and passwords to stop access to other people's directories..
Another option could be to unmount the directories after they log off.
Place the user files in a separate directory, eg /files and mount them as /home/username
when they access them after logging on.
It uses the pre-exec & post-exec commands like mounting/unmounting a cdrom share..