1. You do not need to have enter passwords for machine accounts.
2. Which XP version & service pack are you using ???
3. When you join domain, you have to enter username/pass for your linux box root account.
Are you following the same practice ?
Kindly go thru www.amitsharma.linuxworld.com/how_to_samba.htm
, complete Samba PDC implementation have been mentioned here.