CryptoExpert

CryptoExpert is a free and easy-to-use data encryption application that comes highly recommended.

The application lets you mount high-security virtual drives into which you can drop your sensitive data and protect it with a password. The most interesting part is that the data are decrypted on the fly, which means that if an application has to use the data, it can only read the data in that moment.

This way, your drive will appear mounted and available just like any other (hard drive, DVD, external memory card, etc.) though it can only be accessed with a password.