How can I make a program that's impossible to be killed in Windows?

What you are asking for has implications far beyond what you are being asked to accomplish. For example, if the process cannot be killed, how can Windows shut down normally?

I don't know your boss, so you will have to gauge their level of understanding for yourself. If a user/manager asked me for this, I would have to probe deeper to find what they really mean. My suspicion is that your users want the default close behavior to minimize to the system tray rather than exit the program, similar to most instant message clients, rather then not be able to ever close the program at all.

let me first say, this is a horrible idea.

but...if you must do it, the way I see malicious software and spam hounds do it is to create 3 running processes, one is the main process and the other two will monitor and restart any killed process. it's very difficult for the average user to kill 3 processes simultaneously.

Sounds like a virus.