Microsoft really needs to get out of the "one Windows for all" mentality. You don't take a cargo truck to go shopping, nor do you use a convertible to move houses. There are different types of computing platforms, they need different levels of OS (and UI) functionality.
That means that Microsoft should not try to bundle everything together all the time. That's how they screwed up the tablet market the first time.
At install time on a PC, I really don't see what the problem is in detecting whether or not the user has a touchscreen-capable screen, and if yes, asking the user (yes, you should still ask) if he wants to install touchscreen functionality. If the answer is no, don't bother the user with it again. On the other hand, put it in the list of Windows options that can be installed at any time.
If the user installs the OS without a touchscreen, then buys one and hooks it up, by all means ask him if he's interested in using touchscreen functionality once the OS has detected it, but don't force it.
This is not rocket science. Microsoft already did with disability aids, why should this be so different ?
If the user is installing on a tablet, by all means include the touchscreen stuff - it's needed there and is obviously useful. But Microsoft could still make it an option to uncheck, for the hardcore keyboard-mouse aficionados who would still prefer managing their tablet like a PC. Might not be smart, but there is no reason Microsoft should not allow it. After all, I can imagine very well someone buying a tablet for a specific purpose (say home automation) and bolting it to a wall in the cellar or something.
Yes, it might not be all that smart, but computing today is about letting the user find a solution to fit his needs, not ignoring them and imposing something a big company thinks is better.
I'm convinced that at least 50% of the angst around Windows 8 is because of that restriction to our available choices. We, as a user base, have grown accustomed to expecting things to work the way we want. Right or wrong, it is obvious that companies go against that at their peril.
And Microsoft has long since forgotten how to listen to its users.