In the early months of this year, Microsoft interrupted the development work of more than 8,500 engineers and sent many on special training to improve the security of its Windows operating system. That "stand-down" took nearly two months and cost at least $100 million, Gates said.

"We estimated that the stand-down would take 30 days," Gates wrote in an e-mail sent to more than a million customers who subscribe to Microsoft newsletters and provided to Reuters. "It took nearly twice that long, and cost Microsoft more than $100 million.

[ Read more ]