I believe the easiest way to rename a domain computer account is the netdom.exe command. Netdom.exe is part of the Windows XP Support Tools -pack which is found in the Windows XP Professional CD-ROM. In addition to the computer renaming netdom.exe could be used in various domain related computer tasks e.g. joining, removing, adding and quering.
Syntax of the rename command (wrapped to multiple lines for easy reading):
NETDOM RENAMECOMPUTER machine /NewName:new-name
/UserD:user [/PasswordD:[password | *]]
[/UserO:user [/PasswordO:[password | *]]]
[/REBoot[:Time in seconds]]
NETDOM RENAMECOMPUTER computer1 /NewName:computer2 /userD:mydomain\administrator /passwordD:* /userO:administrator /passwordO:* /Force
Command renames computer1 to computer2. Parameter userD determines the user account that has administrative permissions to the computer object in the domain and the parameter userO the user account that has local administrative permissions on the computer1. The asterisk symbols after the password parameters specifies that the password should be typed with hidden characters when the command is submitted. Without the force -parameter the user is prompted to proceed with the command. Automatic restart is not used in this example but you can set it up with the reboot -parameter.