Dylan Greene has a good list of how Microsoft could stop spyware and adware with Longhorn. I personally like the way that OS/X does it with an integrated sudo type system (the reasons for this I left in my comment on the article). Maybe MS can do better? Course, chances are it'll be some half assed system that you need to connect software B to software B, while not uninstalling software C that no "normal" user (ie: those hit most by adware and spyware) would ever do.
Course, we'll probably have to wait till 2005 or so before it comes out, but on the upside, we'll have from enduring hype, hype, hype from october 26th till the actual longhorn release to convince us it's the right and only way to do it.
Hopefully they'll design an OS that isn't so userfriendly in it's installing of software. Think that'll happen?