MS Explorer window opens on bootup

Well, I do not know how I have achieved it, but since some time MS Explorer window showing the root directory of my C drive opens in the Win desktop on each start or restart of my Win 98 SE system. I have looked through various standard possibilities of this behavior (startup programs, some register entries) but I was unable to find the cause and cure it.

It is no problem, in fact, just a nuisance but still – could anybody point me somewhere to look for the cause of this?