I'm a sound engineer for one of the largest post production and music production facilities in Athens/Greece. I can tell you why the music/sound/film industry prefers the mac over Windows.
Windows is a really great product when you are dealing with non-realtime applications. This includes Internet browsing, text applications (office etc), servers, mp3 encoding, DVD ripping, all sorts of conversions (wav> mp3, tiff>jpeg, avi>mpeg etc.), 3d rendering. Even CD/DVD burning is ok, because of the large RAM buffer used in the burning applications. And since WinXP came along, you can also run Photoshop reliably - it isn't an application that does realtime processing after all and therefore it is rock-solid.
When it comes to realtime prossecing, Windows just can't deliver. If you buy a new PC with a fresh copy of windows installed on it and you install realtime software like Pro Tools, Cubase, Avid or any other software that require resources in real time, you get unexplained error messages. "Buffer size to small" "could not complete the export in realtime" etc.
In most of the cases, artists are artists, they are no hardware heroes. Running complex realtime software on a PC, requires advanced technical skill and lots of time. Artists have less of both.