I think with Microsoft supplying Windows office 2010 Lite or what ever they going to call it for free (albeit with an advert) we will find that there is even less reason for anyone to switch, even from a cost perspective if all a person requires is basic excel and word functionality. I say Linux is Lekker (probably better as a server), Windows wins on the desktop, especially windows 7 /office 2010.
I have been using computers since 1980. (Sperry UniVAC, ForTran)
I believe the Microsoft OS, when combined with Active Directory are WINNERS !!!
The two offer excellent security and ease of adoption where flexibility, stability, integration and cost (especially Admin costs/fees) are concerned.
After using MS products since DOS, and then using Windows through to the current versions, these are currently the best technologies, and are constantly getting better.
Open source may be free but security from viruses are the ultimate challenge. You can't protect open source.