A couple of options. When you sell the PC's, figure out how much it would cost to buy Windows ME/XP/9x what whatever, and install/configure it. Then do the same for Linux. Then when you sell them, say this: x for a plain jane PC, y for a PC with Windows, and z for PC with Linux.
Personally, I have done similar with FreeBSD as the OS, then Blackbox as the wm. I just disabled all the consoles except one, then enabled ssh. Then in blackbox, just enable only the necessary commands in the menu. Then when they say they have a problem, but can still connect to the internet, I can ssh in and work. I set them up so that they can add users and what not. Its not hard to do, you just need to look at what you want/need, and find the best way to do it.