on a computer's hard drive, and nowhere else, you are taking a huge risk - whether the messages are encrypted or not makes no difference. It isn't hackers you need to worry about, it's the risk of losing the computer, or having it stolen. You'll even be in trouble if the hard drive fails - data recovery specialists are very expensive.

I strongly advise you to stop worrying about encryption and start thinking about a proper back-up policy.

try e-Capsule Private Mail, it encrypts the messages as you asked, not the files. But the methods in the above is effective too.