Our consulting is oriented toward finding open source solutions. In practice, this means that we prefer an open source tool, if there’s one with a closed licence and and open source one and they are equivalent. If there’s no open source tool, we can’t suggest it and will say so, of course!
All our in-house computing is open source, we do not use any version of Windows currently, we practice what we bring to others.
However, it is feasible to run mixed Windows and open source solutions, the most well-known tool in this area is probably Samba: http://www.samba.org/
This arrangement is used extensively in the City and Canary Wharf, using Windows desktops and Linux or Unix servers.