Wouldn't change a thing, really. Might make MS even more profit, or at least increase the market share.
Most people I know that have Windows installed pirated it, so technically it was free for them (I have w8.1 pro from Dreamspark, so it was free for me, too).
I know nobody who actually went and bought an actual copy of Windows since XP. Either piracy or use the already installed OS that comes with pre-builts (which has the price included and non-removable).
As for ads, unlikely. I won't say impossible, but I wouldn't bet on it.