If you can get access to the database, try changing the user_password field value in vtiger_users table to:
adpexzg3FUZAk which makes the password as admin
mysql> update vtiger_users set user_password='adpexzg3FUZAk' where user_name='admin' ;
You will also need to set the crypt_type field of vtiger_users to blank (empty value) if you are on 5.0.4 and above.update
mysql> vtiger_users set crypt_type='' where user_name='admin' ;
Now the password for "admin" is "admin"!