Linux is an alternative operating system (OS), which can be tweaked to one's own satisfaction. Many versions are free; the ones that cost generally are also easier to set up, and sometimes (although not often) have some sort of official support line when you hit a snag.
As a general rule, if you're comfortable digging into the specifics of exactly how your computer works, and/or you really hate Microsoft and Apple, you can certainly install Linux as your computer's operating system. (If you are interested, but not sure you want to switch over completely, you can also install Linux in a certain section of your hard drive - a "partition" - and choose on startup whether you want to use Linux for that particular session, or your old OS.) On the other hand, if you want to just start the thing up and go, you might be more comfortable with one of the versions of Windows, or one of the Apple OSes (the latter only if you own a Mac - I don't think they have a version readily available to run on non-proprietary systems, anyway).
Since aspies, as a population, tend to run a bit high on the geek scale, there are a lot of Linux partisans around here... 
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Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.