cannot join domain after upgrade to windows xp professional

hi ,
anyone can help me to solve these problems.
i cannot join the domain after upgrade to windows XP professional .
my domain controller is running windows server 2003.
actually i face these kind of problem long time ago, i keep on asking but no
one can help.
my network connection is working find and i be able to ping my domain
controller . (my domain controller and my clients are in the differents vlan
).
when i try to join it' s keep on said the cannot find the domain controller.
at the 1st i thought is the netbios problem and i try to set up the win
server but the problem still occurs.
anyone have the solution .
thank you