How can I keep my files completely private in Windows 7 (on a shared computer?)

It has recently come to my attention that other users (there are two other user accounts on the computer, all accounts are administrators,) can access all my files, and I can access theirs.

All any of us have to do is go to C:/Users/ in windows explorer and everything is right there. I’ve tried changing the permissions, but so far everything has been ineffective. Even though after changing permissions in properties->security it looks like other accounts shouldn’t be able to access my folders, when I log on to their accounts I can still get to it.

How can I make my documents private?