If you are planning to release the code of your program to the rest of the world, then a demo version is redundant. If not, then you can generate a key when the program is installed and store it in some file. You can check for that file's checksum and timestamps when the file is written to and then on subsequent runs, your program should check the file and refuse to run if the checksum/size or the timestamp changes in any way.

For a user, it would be a bit more complicated, but you could go along similar lines.