Currently, Larson-Green is in charge of Windows products at Microsoft: She led the UI design of Office XP, 2003 and 2007, and she managed the launch of Windows 7 in 2009. Larson-Green recently oversaw the Windows 8 launch, which has been trickier than Windows 7.
This is part of a company restructuring plan that Ballmer is expected to announce next week, at the earliest. In June, reports indicated Ballmer's changes would focus on making Microsoft a "devices and services" company, along with giving Mattrick a larger role.
Obviously that's out of the question now – Mattrick announced his resignation from Microsoft yesterday, and he's poised to take over as CEO of Zynga next week.