Here are a couple of methods.

Option 1.
Right click My Computer and select Manage from the menu.
In Computer Management, click on Local Users and Groups.
Double click on the Users folder in the right hand pane.
Right click on Administrator and select Rename from the menu.

Option 2.
Go to Control Panel and open the Administrative Tools folder.
Double click on Local Security Policy.
In the Local Security Settings console, click on the + signs in the left hand
pane to arrive at the following location.
Security Settings -> Local Policies -> Security Options.
In the right hand pane, right click on Accounts: Rename Administrator Account.
Select Properties from the menu.
Enter whatever name you want in the dialog.
Click OK.
Close the console and see if you can now log on using the new account name.

Note: If you're using the Welcome screen, you'll need to hit Ctrl + Alt + Del
twice to bring up the Logon to Windows dialog in order to logon with this
renamed "Administrator" account.

Courtesy of Ramesh Srinivasan, MS-MVP
How to login as Administrator in Windows XP
http://windowsxp.mvps.org/admins.htm

Good luck

Nepatsfan