if you are letting someone have a quick browse on your computer you can give them a guest session.
on ubuntu this is very easy. click the session menu (the one with log out and shutdown), and choose guest session. it creates a guest user, with limited privileges (eg. they can only read a small white list of the filesystem), and logs them into an X session. when they log out it deletes their temporary home folder. it is pretty hard for them to do anything bad from it.