To be honest, for a setup like that a thin client system might be your best option. Thin clients is the new name given to dumb terminals, where the computer being used is really little more than a terminal for a larger server.
What you sacrafice in speed of the individual client systems is vastly simplified administration. You need only patch the server if a security issue arises, and all clients are automatically patched along with it. You can also run applications over the network and most people will never know they aren't being run locally.
Now, the other catch is that Windows just doesn't really have the capabilities to do this, so you'd probably have to run some sort of Unix. Something that has real network transparency and doesn't care if some file is located on a network server half way across the world, or a local disk inside the server case. Windows just doesn't fit that description, and probably never will without a major rewrite. However, this doesn't rule out being able to teach some computing basics like using a web browser. You just have to use (IMO a far better choice anyway for many reasons) Mozilla or Netscape instead of IE. You can use something like the freeware open source OpenOffice for teaching the basics of word processing, and office type tools. And probably anything else you want to do, you can find a good alternative to the Windows counterpart which will serve you well for teaching the basics. It will also tend to keep costs down quite a bit and ease administrative chores significantly. All while exposing people to something other than Windows and Office so that if they ever run into something else in their life, they won't be hopelessly helpless because they only learned Office.