The agency, which is part of the MDC Partners group, secured the business following a shoot-out against Publicis Groupe's Fallon in Minneapolis.
JWT and Microsoft's agency of note, McCann Erickson, are understood to have been involved in the contest at an earlier stage.
In light of increasing competition from Apple and Google, Crispin Porter will be briefed to produce a campaign that will increase loyalty to Windows' desktop, web and mobile products.