Apple sued Microsoft over Windows, then got
Microsoft to invest and commit Office to Macintosh
for 5 years + $150 million to settle litigation
between the two companies.
It was more than that. Apple was seen as the only 'credible' competition to Microsoft in the OS space. If Apple went under, Microsoft was absolutely going to get smacked with an antitrust lawsuit by the government. As it works out, they did anyway, but I think that a $150mm deal on non-voting shares (which were later resold at a profit) was seen as a no-brainer sort of move.