login problems

I used to log in without a password. Now I downloaded OS X Mavericks and it wants an administrator password to install. Evidently I need such a password since I upgraded to Snow Leopard or Mountain Lion, but my computer was set to see me automatically as a user by the latter (?). So I can't find a login window to try and reset my admin password. Please advise.

Here's some info on resetting password (including the admin one).

log in problems

thanks for info. I've tried that before. Now I see refs to optical drive or use of other computers. A bit cumbersome. Or I can use my Apple ID. I succeed with apple ID changes but I really need to change the original admin password. then I get this:

"To reset your OS X user password with your Apple ID

If you incorrectly enter your account password at the login window three times, a message appears stating "If you forgot your password, you can reset it using your Apple ID". Click the arrow-in-a-circle icon to bring up the "Reset Password" dialog. Enter your Apple ID and password, then click "Reset Password" to proceed."

Now, I don't get the login window (as stated before) . How do I create the login window?