What is the average person's mental model of a computer? Most people don't seem to have a clear idea what a computer does and how it works. There's two kinds of space - hard drive and memory - and only one affects how fast the computer runs. This is pretty important for understanding computers, because it affects your behaviour with it. You won't run as many programs at once if you understand that running programs, not installed ones or big files, that slows down computers.
But is it worth trying to correct that perception? How much does the average person need to know about how a computer really works? It might affect how they use it and how they maintain it, but if they're replacing computers every couple of years and mostly using them for Facebook games and email, how much does it matter? People who fail to understand computers will mostly be using tablets in the near future, and tablets can hardly be misunderstood at all. The big difference that helps? You can only run one app at a time.
Mokalus of Borg
PS - Tablets, though, do still have a memory/hard drive difference.
PPS - That is, you can install lots of apps and not slow it down.