DELL and others dont offer linux or OS/2 or any other operating system with their PC's because there is no market for them.
however, microsoft did not push its way into a monopoly.. it became one because there was no other alternative back then.. so everything began to be built around it. it built up inertia and it all ends up to what we have today.
but it is a monopoly because it does have virtually everyone depending on their products, particularly the Windows OS, just to be able to use their PC's.
Load Linux into your PC.. see how many games you can run. See how many programs, applications, multimedia stuff, etc you can get to run on it in comparison to what you find for windows.
and MS, do not doubt it, has and does take steps to ensure this continues.. its their bussiness to keep making the max profits they can. dont forget they even made windows almost incompatible with netscape a while back

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If tomorrow, out of the blue, someone came out with something much better than windows it would be extremely hard for that new product to replace windows. (unless of course it was a free download *grin*).
its like beta and vhs.. beta WAS better but vhs was already widespread and mass marketed.