By default Windows XP is installed with an hidden Administrator account without password.
To login using this account you can press Ctrl + Alt + Del two or three times once you are on login screen. This will show a new login screen where you can enter the username and password. Login with
Administrator and blank password.
Once you are loged in you can create a new account with administration privileges.
If this doesn't work:
- Try @Nate Koppenhaver method
- Check this question here on superuser
- Check this tutorial
The last tutorial will let you change the Administrator password and then you can repeat the method above and login with Administrator account.