If Microsoft came out with Windows Lite, that was guaranteed to contain no legacy code and was lean and mean, I would pay extra for it. I want something that can run basic software efficiently, and want all the rest of it to just go away.
Nowadays, to get the same thing, I get it for free from open sources. But that is a hassle I could do without.
As soon as you started loading software, you'd end up loading tons of libraries and DLLs containing legacy code to support that software.
There are tools to build a slimmed down Windows distribution (at least for XP, not sure about Vista). You can remove a lot of the useless crap.
Then people load on crappy, bloated anti-virus software, and tons of mostly useless utilities that run in the background. After a rather short time, their computers become more and more sluggish, and uninstalling crappy software doesn't seem to fix the problem, and eventually they go and buy a new computer because the one they have is too slow.
Of course reloading the OS and just the software they need will make the computer they have work fine once again, which is one of the main reasons I think computer vendors quit providing software media with new computers. They don't want people to be able to restore their computers to usefulness. They want you to buy a new one.