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When consumers enter a computer store, in general they already know which operating system they want. The choice they make in the store is thus a choice on hardware. In other words, hardware is the determining feature of the sale. This point was explicitely mentioned in a ruling in France in 2009.
Now, consumers that want to use something else than Windows currently has less purchase opportunities: either they pick among less manufacturers and/or vendors, or they pay for Windows extra.
Consumers that want to use something else than Windows thus have a systematic disadvantage. In other words, competition is biased between Windows and the other operating systems for personal computers.