You typically do not know how long the systems will be used. In many cases they are used to the point that they no longer work. When you are dealing with Windows, that does not take very long. Many windows users actually replace their systems when the software fails--they think they need a new PC. If the systems are older you won't even get latest Windows to run on them--and if it does somewhat run it will be a wast of time for them because the systems will be slow and problematic.
Then there is the licensing issue. Can you distribute Windows to them? Do you have a license to distribute Windows? Are you breaking the license agreement that you agreed to when you originally installed Windows on those systems. IANAL, but you may be setting yourself up for a legal mess with Windows. Read the EULA for details--get good legal advise to be sure. ..Then also how long will Windows run effectively before it gets roached and what about anti-virus software costs, and added costs of applications. When you give them Windows you give them a headache and expenses they do not need--like giving them a free Great Dane puppy from the pound but not as lovable.
So after the 2.5 years of XP support, then what do you do? Endgame in action, more costs.
Now consider the solution using Linunx Mint, or Ubuntu long term support (LTS). When the current LTS support expires the next LTS version will already be available at no charge and supported for another two years. Then on top of that you get the freedom of the GPL, so you can set up a system as a server without the restrictions and costs that occur with Windows. Hell, you can not even set Windows desktop up as a server--it is against the license. But you can use a NIX system any way you want, server, desktop, storage appliance, or whatever is needed. Windows desktop is heavily restricted in what you can use it for--single purpose desktop single user, that is it.
With Linux (Ubuntu, Mint, or others) you can duplicate, replicate, install, move hard drive images around, all you want without having to go through the reactivation mess of Windows. With Ubuntu or Mint, updates take seconds and most times do not even need a reboot. Windows updates take hours, days, and can result in totally screwed up systems--been there many times.