Microsoft invested $150 million in Apple in 1997 though the purchase of 150,000 shares of non-voting preferred stock. That stock became convertible in 2000, and by the end of 2001, Microsoft had converted all of its preferred stock into about 18 million shares of Apple common stock. All of that stock has been sold off.
Microsoft used to own a very tiny amount of Apple stock through a Private Capital Management fund which was bought into by a variety of companies of which Microsoft was one, but I think that PCM no longer has any Apple stock in the fund.