[peterh]

First, set up TCPIP on XP machines as follows:

IP address -> DHCP or static
Netmask -> ditto
Gateway -> DHCP (must point to subnet's router)

DNS -> Points to Intranet's DNS server
WINS -> Points to WINS Server

The computer name must be unique with upto 12 characters long preferably Letters and Digits. Starting with a Letter character.

Client for Microsoft Networks MUST be installed!
File and Print Sharing is optional
Netbeui is NOT required.
Computer account must NOT already exist (deleting existing a/cs)
The Domain suffix must be configured e.g mydomain.com in Computer Name tab under More...

Then you can join the Domain.