Are you logging in as the same user? If you have a separate username it's pretty easy to just set security on the folder (right click on folder -> properties -> security tab) and remove the Allow checkmarks from things like the users/administrators groups, and only leave them enabled for your name. Be sure not to check Deny for any groups that you're a part of though, because Deny automatically overrides Allow...for example if you check Deny for the administrators group, and your user account is an administrator, and you set your personal account to Allow, you would still not be able to access it.
If you're using the same name, you can either encrypt the folder like someone already mentioned, or I'm sure there's tons of 3rd party apps out there that would accomplish the same thing.
edit: Just wanted to add that if you're using separate usernames, and you just set folder permissions, and the other person is an administrator also, they can go in and manually take ownership of the folder and view it that way. Depends how you have your accounts set up.