You’ll need to log into the built-in Administrator account. 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.
Now you will be able to go to Control Panel>User Accounts and see what accounts are on the system and change the passwords for them.