graeme_p - 10:54 am on Aug 30, 2013 (gmt 0)
A lightweight Lniux distro booting off a CD or USB is fast - a lot faster than any large OS booting off a hard drive. I like Puppy, but it needs a bit off work to prevent software installation if running off USB (and you will need to remaster to have a decent browser if running off CD).
An alternative is to use any Linux distro that has a guest login. They cannot install software, and everything they do will be wiped on logout.
Ubuntu can be configured to automatically apply security updates (software updater > settings > updates > when there are security updates > download and install immediately).