If you want to encrypt your sensitive files so that no one can access them without your personal password or decryption key, you have several options. But if you want a free, cross-platform, open source encryption application, try TrueCrypt.
There are some precautions you need to know before you get started as follows:
1). There is a chance that Windows will write the opened cache file in a page file, rather than in RAM, thus exposing your data or key.
2). Avoid using hibernation when using TrueCrypt because at hibernation time the unsaved data is written to hard disk.
3). Content of a mounted TrueCrypt volume is visible (accessible) to all of a system's logged on users.