windows 7 cannot join to domain with this error:"The Network Name Cannot be found"

hello 

i have windows server 2008 R2 as AD and ADC

clients are windows 7 and XP SP3

now my problem is :

when i want to join windows xp to domain i have not problem

but when i want to join windows 7 to domain,i enter domain name,it ask me for user name and password,i enter them,but after some seconds it show me an error :the network name cannot be found!

i tested this with several PCs 

I am thoroughly confused. Please have anyone  any idea on this?

Regards