*sigh*
This really has absolutely no business being in this forum, and to be honest, it has been discussed to death in the appropriate forum over the past ten years, but here's the short version:
The $150 million investment will give Microsoft a 7 percent stake in Apple but no voting rights on the board of directors. Microsoft has promised not to sell the shares for at least three years.
That was in
1997. They sold all their options the following year.
The reason this was done at the time (Jobs cause a HUGE furor with this move) was to signal Microsoft's commitment to the Mac platform - it came with the formal announcement (by Bill Gates, via video conference, at a MacWorld keynote, no less) that Microsoft would be making Microsoft Office for the Mac platform for at least five years.
Basically, Microsoft's public commitment and investment re-established the Mac as a "serious" platform. It was a cornerstone of the Mac's resurgence on the market (along with a complete rebranding as a lifestyle company).