Encrypt and password protect?

Hi all. I have the inlaws coming over shortly. With them, they are bringing their 14 year old son. Now, I know my little macbook here is going to be quite popular with them, and my loving wife won't let me hide it for the duration of their stay. Now I guess I have to "secure" what I can from prying eyes. Does anyone know of any application that would allow me to encrypt files? Preferably freeware, I need to encrypt or put a password on my documents folder, and few other folders on my mac without locking up th whole system. I really don't want to create another user, just protect certain files and folders. Also is there a way to have mail ask for a password before opening? If it's sitting in the dock, closed, I want it to ask for a password when I click on it before it opens.

Log.
Out.
Of.
Your.
Mac.

It's simply not possible to "protect" parts of your account. You can try, but you'll still be vulnerable to all kinds of accidents.

Make a new account. They're free, and it takes all of ninety seconds to set up.

Ok, if I do create a guest user acct, then how do I allow the guest user to access certain files on my acct. I want them to have access to my itunes files, my iphoto files, a few home movies of the kids, etc..... Thats why I was just going to leave my acct open and allow them to use it, while encrypting only certain parts, like, mail, and my documents folder.

The Shared folder (/Users/Shared) is readable to all users (but items placed in that folder can only be removed by the user who placed them there.)

If you place your iTunes and iPhoto libraries (or copies) there, anyone will be able to read them.