Only in WinXP Professional:
To change the Administrator name, follow these steps:
At a command prompt, type secpol.msc to open Local Security Settings. (Alternatively, you can double-click the Local Security Policy shortcut in Control Panel's Administrative Tools folder.)
Open Local Policies\Security Options.
In the details pane, double-click Accounts: Rename Administrator Account.
Type the new name for the Administrator account.
When you have Home Edition:
Because the Local Security Settings snap-in isn't available in Home Edition, you can't use it to change the name of the Administrator account. However, you can change the user name by launching the Windows 2000-style User Accounts.
At a command prompt, type control userpasswords2. Then select Administrator and click Properties. Be sure you change the user name, not the full name.