Operating system policy
From MissionTechWiki
Choice of operating system is a controversial subject. Having a policy may cool the debate. The following statements are broad summaries and you may disagree.
Windows or Linux
Windows costs and Linux is free. However as Windows is used more widely support may also be more easy to come by. Linux support can also be paid for.
What about Mac?
Do you want to support the Mac, or is it not worth the effort in IT support staff developing new skills, and would it be better for the Mac user to learn Windows or Linux skills?
Windows lifecycle
Microsoft don't support their operating system indefinitely. Their website has more information. Do you upgrade PCs and server once the "mainstream" support phase has ended, once the extended support phase has ended, or when you replace the machine?