I don't know if this is what you are looking for but ........
Run the Registry Editor (REGEDIT.EXE).
Open HKEY_LOCAL_MACHINE\ SOFTWARE\ Microsoft\ Windows NT\ CurrentVersion.
To change the values of the RegisteredOrganization, RegisteredOwner, or ProductId, simply double-click on them.
OR
Use this utility:
http://www.dougknox.com/xp/utils/xp_change_owner.htmAnd when you run WINVER, you should see that it's now showing it's licensed to whatever new name you give it.
HOWEVER ....
The user profile home directory name (C:\Documents and Settings\[the previous registry name]\Desktop or something similar) can't be changed. But you can create a duplicate user profile in a different name (preferred name) and remove the old profile at a convenient time but remember you gotta be an admin on your new account obviously.
Here's a website which may be of use to you: Create a duplicate user profile with a different name - Windows XP:
http://windowsxp.mvps.org/dupprofile.htm