USB Safe Guard is a free portable program that will encrypt all of the files on your flash drive very easily, anyone can do it. You can also set this tool up to shred your files in case you lose your flash drive.
To get the program installed onto your flash drive you simply download it from the following website http://usbsafeguard.altervista.org/download.html, then you drag the program onto your flash drive. From here all you do is launch the software from your flash drive, then you should be prompted to create a password. Once you do you are then able to encrypt the entire flash drive or specific files or folders, in which all you do is drag into the interface. This program also gives you options on what to do when someone tries to access your data and does not know the password. You can set it to shred all of the data, leave it alone, plus a few more options.
You can also enter in your phone number or email in case your lose your memory stick. Most other programs like this do not have this option, which is very nice to have when you leave your flash drive at a clients site. This program will work on Windows XP, Vista, and Windows 7.