Well you have two choices you can either use pgp to encrypt the files http://web.mit.edu/network/pgp.html
or just right click on the folder and select properties and select the security tab then highlight the other user(s) that are listed there and uncheck the options as you see fit and if you really want to be more thorough just press the advanced tab and highlight the user(s) and then press view/edit button and uncheck any boxes that you want to prevent that user(s) from doing.