Have you considered teaching Debian Gnu/Linux. This is a Unix operating system, and consists of only Free Software, that is software that allows you to be free, including the freedoms to use, study, modify, and distribute (for any price).
This has may advantages:
Freedom to use in the way that why want.
The system is written for the user. Not for the company that made it, for media companies, for advertisers, or for the NSA.
Ease of use
It is now easier to use that Microsoft's Windows:
It has a very nice windowing system (a choice of several, I like kde, but gnome is simpler), more powerful, and more consistent to use that Microsoft's.
It also has a very nice command-line interface: much, much better than Microsoft's cmd. This is used by intermediates and experts as is much easier, for most tasks, than a graphical interface. Most people use both, as they are well integrated.
Good for beginner, intermediates and experts
Microsoft has successfully optimized their software to be easy to use for beginners, specifically the first few weeks. After that while we, as experts, usually know how to do things, it is a lot of effort. Also there are a lot of surprises for beginners and intermediates.
In Microsoft's Windows , if there is no button to click, then you choose, don't do it, or get visual studio out, and write a C#/VB program. In Unix there as a gradual path from beginner to expert.
Easy to learn
Because it is more consistent, it is easier to learn. When I put Debian on my dad's computer, he stopped phoning to ask for help. He just used it. Before this I could go around to see him, and he would explain what he was trying to do and how he was trying to do it. I would often have to say, you seem to be doing it all correct, I have no idea why it does not work. I eventually got fed up and installed Debian.
Learn it once
Unix improves, but they ensure that it stays backward compatible. What I learnt is 1991 is still (98%) applicable today. However this has not held it back (some people disagree), at least it has not held it back enough to allow Microsoft to catch up.
Lots of software
You think space is big, but that is peanuts compared to Gnu/Linux.
Debian currently has ≈ 43,000 packages (180GB, but you will not need it all, ½GB for a good system, I am a power user and mine in 15GB): These include Office tools, web-browsers, web-servers, software-development tools, etc, etc …
Uses less resource
Installing Apache warns 540 kB additional disk space will be used.
Mono Develop (Like visual studio, written by novel) warns 13.6 MB of additional disk space will be used.
These are big programs, that do as least as much as the Microsoft equivalent. Apache is currently the worlds most popular web-server.
What's a Virus
A virus is something that attacks through vulnerabilities in the operating system. Unix culture is to fix the vulnerabilities.