Edit: Accidently Submitted it to soon, anyways what do you listen to your music on? I generally use iTunes more than anything
The only explanation I can see for iTunes on so many Windows PCs is iPods or the music store. The program is pretty terribly optimized for Windows...at least they managed to fix CoverFlow so that it didn't freeze iTunes for 2 minutes if you selected it.
I used WMP for a while, I don't remember exactly why. It's always been a dance between WMP/Winamp, with the occasional dabbling in stuff like Foobar 2000 and AmaroK.
And recently Spotify as well. Awesome new program that lets you stream music instantly from a huge database. It works surprisingly well and even though it's only in Beta stage there's a TON of music to choose from, believe me. Check it out: http://www.spotify.com/en/