You mean like the transition from OS 9 to OS 10?

I'm sorry but one can't bitch about pushing a UI forward.
Well eventually M$ will hit a wall because they keep using old crappy technology that doesn't work very well with today's OS's. They'll eventually need to do this or else they'll be screwed someday. Maybe not in the next OS release, or the one after that, but eventually its got to happen.
Yes, it won't be like the transition to OS X because there are tons of users to convert unlike us Mac users. So it is easier said than done, but it will still need to be done eventually. If they're going to take 5yrs to make an OS, they could develop a totally new OS with a totally new foundation, give it a compatibility layer to make the transition smoother and build from that and then drop it once the transition is totally complete. Develop an OS platform for the future like Apple did with OS X. They could also keep developers, large corporations, etc up to date at all times, making sure drivers, software titles, speciality software, etc all work with the new OS. Use the 5yrs to make a new great OS and to get software developers and major corporations prepared. Do a gradual transition, not all at once. Do something like a 5yr plan, each year implementing this technology or this year were going to drop this and replace it with this.
I really want Microsoft to make a great OS. I really do. It keeps Apple and other OS developers (Linux) going strong with new ideas and technologies. But, Vista isn't it. I'm sorry, you can say what you want, but its just not all that great. Yes, its more secure and kudos to them for doing that, but they can do better with the time they had.