Is it permissible to install non-genuine Windows for clients, or to buy one genuine copy and install it on multiple devices without activating it, given that the customer must see the device working? And can Windows XP be used without purchasing it after its updates have been discontinued?

It is not permissible to copy copyrighted programs except with the permission of their owners, because copying them nullifies their rights and wastes their money and efforts. The conditions of the program owners must be observed when downloading and copying them. So, whoever buys an original copy for single use, it is not permissible for him to exceed this condition. As for programs whose updates have ceased and there was explicit or customary permission to copy them, it is permissible to copy them based on the principle that "customary permission is equivalent to verbal permission."