MSKB 298593: How To Use the Netdom.exe Utility to Rename a Computer in Windows XP says this:
netdom renamecomputer machine /newname:new_computername /userd:domainname\administrator_id /passwordd:* /usero:local_admin /passwordo:* /reboot:seconds before automatic reboot
A description of the preceding command line is:
Machine is the current name of the computer.
New_computername is the name that the computer is going to have.
Domainname\administrator_id is the network basic input/output system (NetBIOS) domain name and the administrator identification (ID) of a user account that has administrative permissions to the computer object in the domain.
Local_admin is a user that has local administrative permissions, which can be the same account specified for /userd:
The asterisk symbol (*) is the value that is connected to the /passwordd: and /passwordo: parameters and specifies that the password should be typed with hidden characters when the command is submitted.
Time before automatic reboot in seconds is the amount of time before the computer restarts after renaming the computer. If this parameter is not specified, the computer must be restarted manually.
For Windows XP, you need the Support Tools included on the installation media under the Support\Tools folder. For Vista/7, netdom.exe is included by default.