There are a few options available. Here are a couple:
Encryption with Winrar is secure if you use a long password with no dictionary words, both upper case and lower case letters, numbers and special characters in it.
Truecrypt from what I've gathered is probably more secure but most people don't have it already installed, so it's little more time to get it setup.
Whatever password you use to encrypt, make sure to record it on paper, and physically store it in a couple of easy to remember and secure locations that you will not forget. Without it, you won't be able to decrypt and access your bitcoin wallet.