Windows is an operating system that works. Not flawlessly, but then neither does Linux, its flagship open source "competitor". One of the main promises of open source is to integrate heterogeneous systems, and Windows is definitely heterogeneous. But its user interface, however restrictively linear it can appear to Linux users, is within the grasp of most of today's humanity, which is in itself no small feat.
What's my point? A company that wants to be based on open source software must be able to use Windows as part of its standards-based network, not insist on rooting out Windows completely. To start with, all servers should be Linux-based. Open server systems are equivalent if not superior to their closed counterparts. The choice to use Windows on the workstations is made case-by-case, depending on the needs of each user or department. For example, Windows is still needed for tasks like graphic design. Shifting the designers from using software like Photoshop, Freehand or 3ds Max to their still-clunky open source equivalents will result in a massive creative and operational disruption that is unwarranted. Same goes for sound and video design. Other options here include the Macintosh/Macintel as it is based on an open source variant of UNIX and much easier to integrate (TO VERIFY). Of course, the Mac is more expensive but I don't think the markup is only for Apple's brand name.