If security is your number-one priority, and you can stomach using a UNIX-like OS, then I highly recommend OpenBSD (
www.openbsd.org). It's arguably the most secure operating system in the world, it's 100% free (BSD-license) and because it's a BSD, it's rock-solid.
BSDs, and esp. OpenBSD have a bit of a learning curve, so I'd highly recommend getting the book "Absolute OpenBSD - Unix For The Practical Paranoid". You can find it at Amazon, or if you're feeling cheap and criminal, I've seen it on the torrent sites.
I recommend at least trying it in a VM - you won't be disappointed.