Quick Question

I'm leaving for two months, having friends over to guard my house while I'm gone.
However, I do have some privacy concerns and I don't trust people in general, especially with my technology.
Friend or no friend, a risk not worth taking for the sake of anything according to me.

What's the most viable way of making sure you can't access my HDD without say a password or a USB stick as a key?
I'm not too concerned with access via internet, I'm more concerned in person.
My computer wont be traveling with me. So, I want more along the lines of password protection/encryption.
I doubt they'll go through the extra trouble of trying to crack it or even remove the HDD's.
But if the HDD's are protected even if put into another computer, that'd be swell.