Win XP Pro log on Help

I just bought an older pc from a friend, and I can’t get past the logon screen. When I start the PC up it comes to a screen asking for a user name and password. The person I bought the PC from can’t remember what the password is, because it’s been like four years since she set it up. It’s running Win XP Pro. Is there a way for me to disable this whole logon thing? She can’t find the Windows CD that came with the machine, either.
Thanks for the help