The system can only have one "Owner", as in the "Registered To:" field in XP. That does not really matter for anything unless you are using that entry in a script or something. If you want to change it, start regedit and do a search for the old name, replace it with the one you want to put there.
The above post is not correct about user creation, when you create a new user, it does not get administrator rights, you have to add the account to the Administrators group manually. Be default a new user is only part of the "Users" group in the "Member Of" tab.