The user profile folders (C:\Documents and Settings\Username)
_cannot_ be renamed, even if the associated user account has been.
So, your best course of action would be to log on using the built-in
Administrator account, create a new user account, with the username
desired. You can then delete the old user account(s).
HOW TO Create and Configure User Accounts in Windows XP
http://support.microsoft.com/default.aspx?scid=kb;en-us;279783
How to Copy User Data to a New User Profile
http://support.microsoft.com/default.aspx?scid=kb;en-us;811151