Forgotten Password - Can't DL things

Ok..Sony computer using Vista.

I have a main account that I log into everyday. It never asks for a password
to do anything.
I have a 2nd user account just to store a 2nd Itunes account. I tried to
update Itunes on 2nd account and it wont let me bc I do not know password? I
vaguley remember during the computer set-up process entering a password. I
have tried every combination and can't get it. I have checked this site and
am even more confused! So is the 1st account the Administrator? or is there a
hidden administrator that I see messages about? What can I do to the main
account to be able to update stuff on account #2? Thanks in advance!