Now that we can have a look at the source for Windows somebody did on K5 and according to the report MS source code looks much like other peoples source code, complete with expletives, warnings to other programmers, and hack alerts.
Most interestingly, the code documents the much talked about (ab)use) of internal API's in MS's own software - but it also documents that other companies were in on the joke also and that MS developers have taken the effort to preserve compatibility with these misbehaving third party products, so it is only partially fair to accuse Microsoft of abusing an unfair competitive advantage.