Sure, you can install it as many times as you want on a machine. Though, I recommend not using the installer except for the first install, that part would get confused. But AI War doesn't use the registry in any way (everything is in settings.dat, right in your AIWar folder, from your license key to your settings to your achievements and high scores). So just copy the AIWar folder to a second location on the machine, and you're good to go.
Or, if you use the installer for one copy, and Steam or Impulse for another, then that also works just as well. Personally, I have half a dozen copies on my machine. And a lot of people use multiple copies so that they can have one on the official release, and another on the latest beta, or similar.
Thanks for your support, and let us know if there are any other questions!