First off you have to understand that if it runs code it can be hacked. That means that the myth of total security is just that, a myth. ANY system can be hacked. All you can do is make it as difficult as possible so hackers won't want to spend the time trying to get to you. Unless they know something is on your system that they want or they have a grudge against you they will more than likely go to another system that is easier to hack. It's all risk vs reward. Why spend a lot of time trying to get into a system when you don't know what is there, at least for a personal system. Commercial systems are different in that they know that chances are that there is something worth getting unlike a home system.

That said, unless you are connected for extended periods at a time you should be relatively secure running from an actual read only CD or DVD. Running from a USB stick is less secure as it can be written to. Reboot at least daily to make it less likely that any man in the middle type attacks (or anything similar).