I know there are other ways using e.g. oobeinfo.inf but this is a much easier/functional way. No need for any external tools or for native windows tools in order to create the user and change the computer name.
About the addon:
UserCreate.exe will be copied to "%windir%" during TEXTMODE setup and then run at the very beginning of T13 for you to input your Userinfo (RunOnce), pausing setup for 60 seconds or user input. The advantage with this method is you don't need to have an $OEM$ folder.
No predefined input options have been set. For example if all you want to do is create a user, the only fields you need to fill in are Username and Password.
For more info on how to use please visit Cancerface's thread. Credit for making this nifty utility, MSFN topic --> User Creation Tool, for (un)attended user creation
CreateUser v1.0.1
LINK: Foxy_CreateUser_1.0.1_RunOnce_Addon
MD5: E29CFDDC9D24ED9D5FFD0268FF3B011A
SIZE: 107 KB
Description: Create a User during unattended setup
Language: Tested in English, multilanguage support on it's way.
Version: 1.0.1
License: Freeware
Release Date: 2006-05-16
Op System: Tested on 2k/XP/2k3 and works.

If someone has any feature request (regarding the addon that is) or have a better/other way to run this program. Please let me know, feedback is greatly appreciated.
Thanks to Cancerface for letting me release this as an Addon here, and to Mrs Peel's for her excellent layout.
Have fun all!
Edit: 2006-05-14 - RyanVM hosting thank you!
Edit: 2006-05-14 - Will try out/debug different methods of calling "UserCreate.exe" the following days, to be compatible with as many setups as possible
Edit: 2006-05-23 - Finally tested it out on multiple setups. FullUnattended works perfectly.
Edit: 2006-05-23 - Updated to v1.0.1, Security settings disabled can't be set @ T13
Edit: 2006-05-24 - RyanVM hosting (thanks!)