Just to second what has already been said, TrueCrypt is a fantastic program. It gives me the freedom to keep sensitive information on my USB drive without worrying about losing it. Its truly the best on-the-fly encryption utility available right now....and the fact that its free and ultra portable is nice too. Im using a combination of 3 programs to keep my portable data secure right now.....listed below.

TrueCrypt - For data stored on my hard drive or on my USB drive
Thunderbird w/ Enigmail (PGP add-on) - Sending Email securely
fSekrit - For creating quick self decrypting text files encrypted w/ AES