Joining the Domain

I just installed Win2003(Evalution Pack for 180 Days). I installed A/D and configure DNS and already have registerd (domain name).
I’ve created 2 user/name and passwords for the users in OU.
How can I join my Worktations.
1. Win2k.
2. Win XP
Win XP can Ping to (Server) but server can not ping to (XP).
Win2k can ping to server and serve can ping to Win2K.
How to join Domain.
Any help will appreciated…Thanks