joining a SBS 2003 domain

i have a SBS running on SBS 2003. I am trying to make a PC with Windows 7 and another one with a vista to join this domain by right clicking on computer-properties-change settings-change on the computer tab-domain name-ok. however, the computer refuses to join the domain saying that the domain name server is not found, please check the domain name and type it correctly. This is despite the fact that I have been able to hook on other PCs with windows XP to the same domain with the same procedure. The IP scheme is also right. probably Im skipping some steps or settings. please help.

I had that issue than I discovered it is best to have al machines use the ad servers as the dns primary dns server and your original primary as a secondary dns server.