Password Issue

Today, I had my MacBook Pro rebooted after it petered out last night from a lack of hard drive space. As a result, I had to reestablish my administrative password. I did so with the Mac OSX disc, and I was able to log in fine. I then made sure to alter the settings in the Keychain Access utility so I can install and delete products and such. However, when I tried to install Microsoft Office or update Itunes or delete a folder I did not need, iTunes would not recognize my new username and or password?

Can someone explain? Is there an amount of time before the new account is fully in place? Did the guy who helped fix it screw around so I could not mess with the settings? This is all very frustrating.

Can anyone explain this problem? I'm sure it's been asked before and I apologize, but I'm still fairly new to all of this.

Anyone?

I'm totally confused on why you had to reset your admin password just because your machine rebooted?