All users (including the admin) are stored in the database in the ost_staff table. Presuming that you have access to the database with another username/password that has (or can have) privs you should be able to run a MySQL query to change whats in there. It is stored as an MD5Hash iirc. Here is the query:
UPDATE `ost_staff` SET `passwd` = MD5( 'newpassword' ) WHERE `username` = 'username';
Obviously change newpassword to what you want the password to be, and username to the admin account name. This same query can be used for any user with those changes.
How to run a MySQL query varies depending on what OS your running under of course. :) I personally use a product called Navicat Lite at home. At work I have MySQL WorkBench installed. If you do not have a priv account in MySQL (like root) there are several good articles on the net about how to recover that password.