The Miami agency, part of MDC Partners, won the business after a shootout against Publicis Groupe's Fallon in Minneapolis.
It is understood that WPP Group's JWT and Interpublic Group's McCann Erickson also pitched, but did not make it to the final shortlist of two.
Crispin Porter will work on developing a global advertising campaign for Microsoft's Vista and Live platforms with a focus on mobile.
Microsoft kicked off the review in November. McCann Erickson, the incumbent agency on most of Microsoft's business, is thought to have been asked to pitch.
At the time a Microsoft spokesman said: "We are soliciting input from various agencies, including McCann Erickson, for a new assignment."
Media for the new task will be handled by Universal McCann.