Forgot the link. ;-)

To the OP:

The smartest thing you can do with a used computer is to format the drive
and clean install Windows.

http://michaelstevenstech.com/cleanxpinstall.html - Clean Install How-To
http://www.elephantboycomputers.com/page2.html#reinstall_Windows - What you
will need on-hand

However, here is the answer to your password question:

In XP Home, boot the computer into Safe Mode. Do this by repeatedly tapping
the F8 key as the computer is starting up. This will get you to the right
menu. Navigate using your Up arrow key; the mouse will not work here. Once
in Safe Mode, you will see the normally hidden Administrator account. The
default password is a blank.

In XP Pro, you do not need to go into Safe Mode. At the Welcome Screen, do
Ctrl-Alt-Del twice to get the classic Windows logon box. Type in
"Administrator" and whatever password you assigned when you set up Windows.

If you reset the built-in Administrator account's password in Home or have
Pro and don't remember the password, use NTpasswd to change the built-in
Administrator account's password to a blank.

http://home.eunet.no/~pnordahl/ntpasswd/

Then go to the User Accounts applet in Control Panel and set passwords that
you will remember and make other desired changes.

Malke