From Apple's 2003 10-K stock filing:
"In August 1997, the Company [Apple] and Microsoft Corporation (Microsoft) entered into patent cross license and technology agreements. In addition, Microsoft purchased 150,000 shares of Apple Series A nonvoting convertible preferred stock ("preferred stock") for $150 million. These shares were convertible by Microsoft after August 5, 2000, into shares of the Company's common stock at a conversion price of $8.25 per share. During 2000, 74,250 shares of preferred stock were converted to 9 million shares of the Company's common stock. During 2001, the remaining 75,750 preferred shares were converted into 9.2 million shares of the Company's common stock."
According to a blog (sorry, couldn't find any better documentation of this, but I haven't found anyone who doubts it, either):
"All 18.2 million shares were sold by Microsoft in 2003."