I know this is a tad late but um...

www.openbsd.org

OpenBSD is the most secure OS around.

maybe you've heard of openssh? these d00ds developed it
also their program "pf" packet filter is pretty freakin solid.

just stick this machine between whatever it is you're trying to protect, then the only fault is really up to the admin through bad configuration.

linux is also secure, but has open holes in the default install.. open is very secure, and a light OS.