Store (Admin) hacked

Hello,

Recently, I tried to log in to the admin-section for one of my webshops. I could not log on and displayed the admin table in PHPMyAdmin. There were two records but neither of them was with my username. I then deleted these two records, added a new one with a blank password and went to the password forgotten page, entered my email address and received an email containing my new password.
My Admin site is not fully protected (Admin directory still is called 'Admin' and I do not use https and so on but maybe you can get some usefull information from my admin_activity_log about the hack attempt.
I do not know about known security issues on how this attempt was made, so maybe it is a security issue that has been addressed by the recent updates to 1.3.9d.
I will upgrade to 1.3.9d as soon as possible but I thought I should mention it to you anyway.

I use ZenCart 1.3.8a
It has not been upgraded from an earlier version (fresh install)
I have PDF order center, Super Orders and Dutch Language pack installed directly after installing Zen-Cart.