There are a lot of RPGs out there, and most of them are pretty dense.
I've heard good things about the Mouse Guard system. It's based off of a graphic novel series about a bunch of mice that are knights. It's a kind of kid friendly theme, but that's what helps make it so easy to understand. It's...
You may want to look into Ubuntu. It's honestly very user friendly. It's free and has great features. You might want to update your question with computer specifications. Depending on how powerful your computers are, you might just go with Windows 98. If they can handle something a little more...