windows vista is actually kinda terrible imo...
been using it since beta 1 and heck even had a virus attack on it!!
the only way that windows would be stable is that the drivers are all proper drivers, and not having intermitten or faulty hardware.

about mac... i have used it only a few times.. i like the overall design of the ui and even the external hardware looks. I'll buy a mac just for the looks.
oh ya.. bootcamp or parallels do run windows vista on them too.

i do use linux also... it has way much better eyecandy than vista. and i wont judge osx about the eyecandy. but linux based system lack the iteroperabilty and support for hardware. so linux users are stuck till someone comes up with some sweet drivers

if microsoft stoped making windows. all the world would shift to osx/linux
this means all the virus, anti-virus, technician's, the entire IT world, games and the rest the things that make up our daily life.

so what i'm saying is this. mac's are save from viruses and instability just because it's not consumed by the mass market.
Microsoft has instability problems because they just have to cope up with thousand's of things designed to run on their system.
and one more thing. Microsoft just wants to make more money rather than easing someone's work