here is a rule of thumb that we use:
If you don't feel comfortable with other's knowing, then don't do it.
This is what we practice here in my office. It's just the easiest way to make sure your not crossing a line. If you don't know any better, either from your lack of training, or you just don't know, there are actions to resove that.
For the home, If your tyring to hide from Mom and dad/wife/girlfriend/friends, no matter what you do, they will always be able to find that info if they spend enough time.
You can create a password protected Profile in XP, but any admin of the machine will be able to see your info. Same goes for encrypted files. Problem is that you can't encrypt everything to keep it secret. Best option is to always clear your trail from behind, so after you are done doing what you have done, clear your history, your IE cach and cookies. That is the only way to stop people from seeing where you were. now, there is still a cache in windows that contains that history. So be sure to clear recent documents too, or you can type run http:///....
and see recent webs.
In the business/school world this is all tracked off the machine and through something like a proxy server or web monitor, which means you can't hide your trail. But most every where else, you don't have that, so you can clean up from behind your self too keep it secret.
Hope that helps you...