The "yellow sticky notepad with the new password left on my desktop" is definitely a violation of security and the boss of any company should know that.
He might have a reason that he has to access your PC and he might have a reason to ask the IT to reset your password. But it is inexcusable that he put that new password in the open. He should have informed you that he had to reset the password and then IT could have reset it again and give you a new password - which your boss does not know.
Edit: About using the user account or a different account: Yes, IT should be able to get data from that PC using a different account. But in real life it is often just so much easier to logon with the "normal" user account. I think that is a question of company policy and the circumstances and not easy to answer.