Just use something like TrueCrypt with a hidden volume. They'll see you have a big encrypted chunk of your hard drive if they give you a thorough inspection. You put in your passphrase, and it'll open up a partition full of innocuous stuff - fake corporate data, complicated looking Excel files, etc. You neglected to check the box and enter the passphrase for the hidden partition, though, so it just appears as free space. Plausible deniability.

http://www.truecrypt.org/docs/?s=plausible-deniability