Um, the Windows division of MSFT brought in $19.2B of their $78B in total revenue. I'm no expert, but giving up 25% of your revenue is no way to run a company.
Even more important, Windows brings in almost half of their operating profit. I'm going off the 2010 10-K, Windows was ~$12B of operating profit, MSFT Business Division (i.e. Office) was about $14B of operating profit ... out of $27B of total operating profit.
As you would expect, the monopolies bring in the huge margins. The other stuff just earns a normal return on capital.