The tool I've used several times, with various Windows versions, is
ntpasswd. It won't tell you the passwords, but will let you blank or change them.
Also, besides the user account you have forgotten the password for, there is the built-in administrator account that you may have never set a password for (Windows setup makes skipping that pretty easy). If so, that would be the easiest way in.
The Administrator account is not displayed on the welcome screen, but you can press Ctrl+Alt+Del twice and type "Administrator" username at the classic logon prompt for XP Professional, or boot in safe mode for XP Home.
