Is possible to protect files from being copied if you are the administrator of a machine? I heard that this behaviour is possible: one software developer sells his software in such a way. He installs it on every client's computer to prevent the software from working on other computers or to prevent it from being physically copied. How can this type of copy-protection be implemented? Is it cost-effective to implement if the software only costs about $100 for all copies to be deployed across the client's company?
This is not a problem that cryptography can solve. This is problem that can only be addressed with the Operating System's Access Control by removing read privileges to the file.
Or put another way, DRM is a fundamentally flawed approach, and will never work.
You can not prevent to copy a program if it is executable, because executing a program means copying it into the memory of the computer. But you can use software and hardware dongles. Simple dongles use the MAC address of the computer.