Best way to encrypt data

["Gary Hennigan" <glhenni@sandia.gov>]

What's the best way to encrypt data on your hard drive such that if it
is stolen you have some assurance that they can't get at the data? My
understanding of how cryptography works is limited, but it seems that
public/private key stuff, like GnuPG, wouldn't be sufficient for this
because your secret key is stored on the hard drive and if someone
gets that it's all over. 

I suppose you get generate your key, decrypt the file, and delete the
key, but that seems like a difficult process if you regularly need to
access the data. Any suggestions?

Gary