Microsoft today said it will make Windows free of charge for phones and tablets with screens smaller than nine inches, a move designed to help boost the companys market share. The announcement comes alongside plans to let developers make universal applications that work on all devices running Microsofts software — both Windows Phone and Windows. That feature is headed to Windows 8.1 as well as Windows Phone 8.1, which was also detailed on stage and is arriving on mobile devices in the next few months. Microsoft has been experimenting with a free, or low-cost version of Windows 8.1 with Bing, which includes a handful of Microsoft apps and services aimed at Windows 7 users.