I don't remember the details on how serious the favoritism is, but I heard from a co-worker of mine that in Windows 7 they do everything they can to get you to use IE. Probably because they know they have a shitty product so they use their OS to force you into using it.
On the other hand, I sometimes get similar troubles on my school's website in XP. I think that's the fault of the webdevs though.