DH recently has to de-bug his father's computer, which was not then password protected, because they had some friends stay over for a few days, and then the computer would not let anyone open, well, pretty much any program. DH had to do a "nuke and pave" because it was messed up that badly. He checked back in the surfing history, and found lots of 'pron' sites, and lots of trojans, virii, etc.
So, after the nuke and pave, he set up an admin account, his father's account, and a guest account as a restricted user.
Hadn't done it before because none of the family had ever caused this issue to come up.
And if you want to, you can always tell someone they need to get off your computer. Now! But the password is always good. All of our systems, desktop and laptop, require a password to get onto them.