If you password protect a PST file then forget the password, you need to use a password-crack program to remove the password. But before you buy one, you can try using an old utility Microsoft developed many years ago for upgrading psts. It's called pst19upg.exe and in the process of upgrading, it removes the password. This won't work for Outlook 2003's Unicode pst format, but should work for all older versions.
Instructions are here:
Published February 16, 2006. Last updated on December 8, 2011.