cost of ownership = 0
who buys windows anyway for 100 euros? everybody gets a free copy anyway these days.

user friendliness
windows is more userfriendly than linux. go ask those millions of users worldwide who use it.

ease of general installation
windows beats linux hands down.

hardware/peripheral detection and automatic installation
windows beats linux again.

availability of third party software
windows trashes linux.

quality and variety of software available
windows wins again.

stability:
linux trashes the old windows 95, 98 etc...
however starting with xp, there is not much difference anymore.
cannot say much about vista, haven't played with it yet.

and yet I am a linux user. I use mandriva. however unlike the other linux users, I am not blind to its imperfection.

only by being aware of one's weakness can one improve oneself.