It takes about 30 seconds to break an administrators password on XP, 2000 and NT (it can be done with a W98CD in an emergency ;-)) ). There are also loads of freely available progs, such as *ahem* John the Ripper, that can do the job faster. It is also fairly easy to make yourself the administrator and bypass the main admin...*cough* safe mode *cough*. All this info is freely available on the internet so it is not like I'm selling the family silver. ;-))
If people are paranoid about others seeing the drivel that comprises the bulk of info on home computers, then they should at least know this.