I support a lot of WordPress systems and find a common occurence to be adminstrators forgetting their password.
In order to reset the admin password I run the following simple SQL query against the WordPress database using MySql query browser.
Replace your_database and your_password as required.
UPDATE your_database.wp_users SET user_pass = MD5('your_password') WHERE user_login = 'admin';
Note: You can of course use the snippet above for any user, but if it’s not the admin password requiring a reset perhaps using the login area will be easier.