Help re automatic prompt to change password

Perhaps I am suffering from selective amnesia, but I honestly do not recall
setting-up a password when I installed Vista (clean installation, not an
upgrade). I have never been prompted by Vita to supply a password at boot-up,
log-on, or at any other point.

Now I am receiving an automatic message that my password is about to expire.
When I go into the normal password reset screen, Vista insists that my
Administrator account (the only account on the computer) is, indeed, password
protected and insists that I supply the old password before it can be reset.
I have tried to use many different passwords (from my pool of typical
passwords) and they are all rejected.

So, am I crazY? Did I install / set-up Vista without a password? Or, is that
impossible and I have simply forgotten the password? In either case, will
Vista lock-up and become un-useable as soon as the password reset countdown
expires? Is there some way to halt the automatic password reset function?

TIA,
Danny