I would recommend against ReactOS, it is still alpha stage, and even some of the OS features don't work properly (such as configuring the taskbar/start menu/file explorer.) It's a nice idea/experiment, but still has a way to go before it is ready for regular users to use without getting frustrated with it.
On the slower machines, is Windows a requirement or could you use linux instead? Something that does not have all too much for system requirements such as Xubuntu, Mint LXDE, or Lubuntu, or even something lighter like Tiny Core? Using Win9x seems like a bad idea because it opens the users to so many security risks, and no way to update or patch them.