Ok, lets talk buisness:

Curent yahoo market capitalization: 36 billions $ http://finance.google.com/finance?q=YHOO 

Current Microsoft Cash: 36 billions $http://finance.google.com/finance?fstype=bi&cid=358464 

Hardly do I believe that Yahoo shareholders will refuse to proposal of 50 billions $. Half of them maybe payed with cash and the rest with stocks. Microsoft will remain with about 10 billlions, which doesn't make any problem (they are profitable company for decades).

Although Yahoo server run non-Windows, that not too important here, since MS buys them mostly not for technology, but for market share and wide range of services, including all the flickr, del.icio.us, widgets etc.