REDMOND, Wash. -- Jim Allchin, a senior Microsoft Corp. executive, walked into Bill Gates's office here one day in July last year to deliver a bombshell about the next generation of Microsoft Windows.

"It's not going to work," Mr. Allchin says he told the Microsoft chairman. The new version, code-named Longhorn, was so complex its writers would never be able to make it run properly.

The...