Onan the Clumsy,
The Admin Password is something you are prompted for when you first install WinXP on the computer. You do not ever want to leave it blank, since a system without an Admin Password is like leaving your front door open for hackers to come right in.
When you bought the computer, they should have either given your the Admin password, or been available when you need the password, to give it to you by phone.
Take Care,
Richard