There are a couple of encryption packages I have used with my Windows PC and my Apple MAC which seem to work. BUT BE WARNED, if you forget your password, there is no recovery system.
For the Apple MAC, use PRIVACY BOX available from the Apple Store (not free about £1.50), screen shot below.
For Windows use SAFEHOUSES, available from http://www.safehousesoftware.com/ screen shot below
They both work in a similar way, but follow the instructions carefully....
Let me know if you use either of these and how you get on with them