Install XP and all the applications that you want to make it useful. Install ALL updates so that the machine is not vulnerable to the many bugs that have been found in windows over the years.
Once the machine is perfect install SteadyState on it. SteadyState is tailor made for situations like this. The state that your machine is in that you turn on SteadyState is captured. Someone can come in and install all the scumware in the world and as soon as the system is rebooted, the machine will be right back to how you had it when you turned SteadyState on.
Check it out..
http://www.microsoft.com/windows/products/winfamily/sharedaccess/default.mspx
