It doesn't matter whether they have a concept of it or not. They're still paying for it. The average user pays for an OEM copy of windows. By far the majority of computer users buy a premade computer with a copy of Windows pre-installed. How many millions of personal computers do you suppose Gateway and Dell and HP and Toshiba and Acer and IBM and Sony, and all the generic computer stores, etc. sell? MS gets a cut of each of those. If MS makes $75 for every one of those computers sold that's 3/4 of a billion dollars for every 10 milllion computers sold. Worldwide.
And, of course, if everyone is using MS at their desktop then every developer is developing for the MS platform. There is a direct correlation between how many desktops are running MS and how many development technologies MS sells. The more people that use Mac OS, the more developers start developing for it instead.
And Windows is, of course, the tool MS uses to Monopoly their way into any market and crush out the competition (e.g., Netscape, WordPerfect, etc.)
Windows is MS's bread and butter.