You can't really assume any prior knowledge. Nothing is too trivial. The challenge is to put yourself in the shoes of someone who is a novice, and not take anything for granted. We use sample groups from family and friends and other individuals, people who have never touched a computer. The best way to put yourself in the beginner's shoes is to watch as the courses are tested by these kinds of people.
Concepts are tougher to teach than skills.
If you are teaching the concepts of hardware or software, in general, it is much tougher than teaching someone how to find a menu or what button to click. We use a lot of 3D graphics in that regard. We try to avoid getting too deep into technical issues, because that can turn people off. They just need a basic understanding of the computer environment to be comfortable using it. The multimedia elements can really work for you, but they can also work against you.