I've never figured out how Microsoft forced companies to buy their products. Did they use a gun? Or did they say "If you want this level of discount, you have to purchase this package of licenses"?
Practically. What Microsoft did was to tell x86 manufacturers that they must buy a Windows license for every x86 machine they built even those that were not meant to have an OS when shipped. Also these contracts prevented the manufacturers from making "Dual Boot" machines. So if Compaq wanted to make a BeOS machine, they still would have to buy a Windows license from Microsoft. If they didn't, Microsoft would yank the all the licenses from Compaq. This what anti-Windows folks called the "Microsoft Tax".