I think what Windows has done is made computer users less intelligent, less intuitive, and MS likes them this way. Ignorant computer users will stick to what they know, they have brand loyalty whether its a good product or not, and they have learned all sorts of little tricks and quirks along the way so that when something breaks, they might be able to fix it.
Back in the day, you used to have to have some pretty extensive knowledge to operate a computer, and this could even be true for Windows 98, though it would boot into the GUI automatically, it was still running on top of DOS, and if something happened to your autoexec.bat file, you might have to mess around in DOS again to get it working.
If the common user spent more time learning about all the modern advances in computing, I'm sure many of them would at the very least have a dual boot system. Its true though, Linux is not 100% capable of replacing the common users desktop for the simple fact that they wouldn't know how to install software no matter how easy you made it. Modern Linux distro's are getting there when it comes to software distribution and system upgrades, but sometimes you do have to get your hands a little dirty in the terminal... as you once did in DOS.
I installed Ubuntu Linux 7.04 on my parents computer and turned it into a dual boot machine. I then rebooted into Ubuntu, and made sure that everything was up to date, and the applications they would need for their limited use would function. They were already familiar with the Firefox icon, so they knew they could check their email. They were also familiar with the concept of a "Desktop" so they could easily save email attachments to it, and then open them with whatever application loaded on the screen when they double clicked it. They Knew they were not in Windows while using it, but they didn't complain, and they actually said that they liked the ease of use, and the "smoothness" of whatever they were using.
That was all the proof I needed that Linux could be quite useful for the common user... especially if you consider that the common user only really uses a computer for word processing, solitaire, web browsing, and web based email services.
Some gamers previously posted the issues they've had while trying to use Wine to play their Windows games, which is a true downside to running Linux as your sole operating system. However, if the market share were large enough, it would be just as easy for the programmers to develop cross platform games which could then even open up the Mac world to even more video games as well.
Which came first? The chicken or the egg? In order for Linux to stand a chance on the common users desktop, we must first have consistent and simple methods for the user to install and run programs. Not to mention that programmers need to also take an initiative, throw out those god forsaken C# and
Linux, like Mac OS X, will not be replacing all Windows installations... but I think more users will be willing to take the plunge and delve a bit into Linux or Mac OS X. People have been asking me what a good laptop purchase would be, and I've been pointing them to Mac. I cannot support Vista. After all the BSOD's I've encountered simply trying to get them on a Wireless or Wired network, as well as that awful and useless UAC.
I'm hoping that since computer users are getting younger and younger by the day, that they will be savvy enough to install the OS of their choice on their PC's and boot into whichever suits their needs.