Best Free "remote Control" Software For Managing A Few Dozen Computers? (Nonprofit)

I have a client who runs a non-profit organization that puts computer kiosks (running Windows XP) in public areas where low-income people can use them for free. They take old computers, put them back together, and then install them into local grocery stores, gas stations, etc.. where people can use them for free.

This is non-profit work, so their budget is pretty much spent on hardware and upkeep of the computers. (ie: internet connections, Windows licenses, etc.)

They have about 3 dozen computers now, and managing them is becoming more and more time consuming and difficult so they have asked if I can make some suggestions to make things more efficient (easier).

All the computers are built with a hidden partition which contains a drive image. If the computer gets too "screwy", then they can simply re-image the machine and "restart". However, this requires a trip to the local computer, which takes a lot of time, and all of this is done by volunteers...)

One idea I had was to use "remote control" software so the volunteers doing the work don't have to actually go and physically see the computer when debugging a problem, updating software, reimaging the drive, or whatever.

A typical problem might be that the store owner where the machine is deployed will call and say, "It's not working any more". So now a volunteer must go and visit the computer to see what's wrong... but with remote control software they could simply login remotely, fix whatever is the problem, or if needed start the drive re-image.. without ever needing to visit the computer. (Save time!)

So... I've done quite a bit of research on remote control software, and there seems to be lots of "for pay" options. However, the organization just does not have the budget to pay for remote control on a few dozen computers. (Also, they are expecting to roll this out in multiple cities soon, so the solution must be scalable, and paying per computer just isn't realistic given their budget.)

I found the "VNC" remote control software, and that seems to be perfect. It's free, and seems to work well.

However, there are no less then 10 different "flavors" of VNC available, so I'm hoping perhaps a few people here might be able to recommend which one is the best solution. tightVNC, ultraVNC, miniVNC, there are even a few pay (one time small fee) projects.

I have no idea which one is best for them, and I just don't have time myself to investigate and test this all out to make an informed decision.

I'm hoping you can help me narrow down the many choices to either one or two options.

Here's what it needs to do:

- Needs to be "always on" so that the remote control software works immediately after the computer boots. (ie: Even if nobody logs in, remote user should still be able to see the screen and do a login.)

- Be impossible for computer users to remove.

- Be able to take control of the machine without anyone being physically present to click "ok" prompts or accept incoming connection, etc.

- Work with remote machines that are in a LAN with an unknown IP. (Some of the computers are plugged into an existing LAN so that they can use the existing internet connectivity.) In this case, the remote machine does not have a public IP, and the current IP might change after each reboot. The remote control software must be able to "see" the local machine, even if the IP changes, etc... (Maybe some software that runs on the remote machine that "sends" its IP or updates a hostname to an intermediary server every few minutes?

- Allow file transfers between remote machine and controlling machine.