Alt Text for System Screenshots

I got into a discussion today with someone developing a training manual/users guide for our program, about appropriate alt-text for a screenshot of a system.

Let's say you have a brand new program, and you have to get everyone up to speed on its functionality. You have a step by step guide of how to do the XYZ task. And you throw in some screenshots to help callout buttons to give the viewer an idea of what they will be looking at. But, what kind of alt-text do you put for that image?

I follow the rule of "what do you want the learner to see" in the image. If your tasks are highlighting the tabs on the screen, then maybe my alt-text is "Main page, with the following tabs available - one, two, three, four".  For me, its providing enough context so that the screen reader user "sees" what a sighted learner sees. If its an eLearning course, sometimes I will have a hidden text box to provide more information or context.

I'm curious how others have handled writing alt-text for progam screenshots. Please share!