Tks for reply,
Done that ... i now have two account within root group ( root and laurentpi )
( laurentpi is my unix and XP account )
I did try to add my computer to the domain again, thus i'm using 'root' account.
When validating, it says "Username cannot be found" even if i try with 'root' or 'DOMICILE\root'
just in case, password for root account is the same for samba and unix.