well, do the research, it is true - microsoft has never written an operating system by themselves
they bought dos and asked ibm to write os/2 but didn't want to market it when it was about to be finished. their latest - the NT - is an interesting story, because they had many companies together writing the os for them (and for that reason nt initially supported many cpu architechtures, the feature later dropped by microsoft). in addition to a great deal of vax code there must have been a lot of os/2 code because ibm was also developing it at that time, plus one of the companies involved were digital, which brought a lot of insight in memory handling.
microsoft is not a software company. it is a marketing house. it does pretty damn good job in marketing somebody else's products as its own