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hardware is the very latest or several years old, it can be installed on them all and set up to work and look the same for all the systems. They also have GUIs that look like Win 2000, Win XP, and Win 7 if you want to make them familiar looking to your users.

Edit to add - because you can have ALL the machines looking the same it won't be obvious which system is a recent replacement and which isn't. Nor will you have a learning curve for them to use the new OS as it's the same as the old OS, just newer hardware.