Here is a sample smb.conf file:
#start
# Global parameters
[global]
workgroup = your-domain-name
netbios name = just-as-it-sounds
encrypt passwords = Yes
update encrypted = Yes
min passwd length = 4
domain logons = Yes
os level = 65
preferred master = Yes
domain master = Yes
guest account = any-account-with-minimal-access
admin users = self-explanatory
hosts allow = X.X.X. (a bit of security)
[netlogon]
comment = Directory logon path
path = /usr/bitch/netlogon
[homes]
comment = User Home Directory
path = /usr/bitch/home/%U
read only = No
browseable = No
#end
Also you will need to patch your registry on the XP machine:
Check out this site for info on that:
http://www.wlug.org.nz/RequireSignOrSeal
Now just as a note, if you just want to connect to the internet through the Linux box you don't need Samba, you just need to set it up as a firewall/router.
Let me know if this helped.