Unauthorized change of Joomla admin password

I have Joomla! 1.5.2 Production/Stable [ Woi ] 22-March-2008 22:00 GMT. The problem is the admin password somehow changes by it self (or by some virus, etc.). I use only one administrator account, so nobody else can change it from Jomla. The password change happens about once or twice per month. And when that happens I have to manually change the password with MySQL command: UPDATE `jos_users` SET `password` = MD5('new_passwd') WHERE `jos_users`.`username` = "admin";
 
How to disable option that some program can change that password without my permission?