Join WinXP Pro to W2K Server Domain

As network supervisor I need to join a new WinXP Pro Desktop-usually very easy. Performed usual steps to configure for end user – among other steps, uninstalled Norton, turned off Win Security Center Firewall, entered domain user name and password in Network ID, etc. then plugged in network cable, opened my computer to properties to join domain on server. Surprised to see LAN already connected, and able to ping server and other network machines, can’t ping it back. But tried to change from workgroup to domain, and join – mssg – can’t detect domain, make sure spelled right. Used other network machine as example to validate names,spellings. Spent two hours on phone with manufacturer techie-stumped. Ran Diagnostics-all okay. Suggestions?