Your mother’s operating system
As a computer knowing fellow human being it is my ban and blessing to be the answerer for every computer related question.
Short:
Why not create a user interface that is so simple that even your mother can understand it? And even you can talk her through over the phone? Then introduce more functionality as the user’s knowledge and self confidence rises.
Long:
Create a user interface to a computer. Let it run a simple web browser and a simple email client. Let it have one and one only folder to store files. Let it be easy to setup for remote control. Maybe even maybe stop the user from changing things to start with.
Bare with me for a few more lines if the above solution is too rough.
Let there be only one way to start things. Skip double click. Skip drag and drop.
Now let the user use this for a while and become comfortable with reading email, chatting, surf the web and print pictures for the camera.
Add more functionality. Depending on the savviness of the user – introduce drag and drop or the context menu. Introduce more programs or the possibility to format emails. Allow more ways to do the same thing.
By the time of writing I see no way of doing a transparent change from Your mother’s operating system to Windows or MacOS or KDE but one should be able to introduce the ideas, allegories and words from a full grown OS GUI.