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Apple was in bad trouble a while back and Billy Boy helped them out by buying a chunk. I would assume there was some sort of deal but I would imaging it would have regarded Office rather than the OS, as MacOS at the time was a joke. I beleive that Microsoft owns ~12% of Apple, but it was a fair while ago and I can't remember what the figure was. >>
Apple was in trouble, but they did not need Bill's money. At the time they had over 2 Billion in the bank. Microsoft bought 150 million worth of Apple stock (I believe they later sold it for over 500 million dollars, so they don't own any part of apple anymore). Also, their share was very little (nowhere near 12 percent) and were non-voting shares. It was really just a symbolic gesture that Microsoft was dedicated again to making applications for the Mac and that Apple would not make a browser to compete with IE. Since the deal was struck, IE and Office for the Mac improved greatly.
Some people say that the 150 million was to settle lawsuits because apple dropped all pending lawsuits soon afterwards.