Software security for data base program

I want a simple way of safeguarding a sotware program ( a data base) that I'm giving away to my customers. I only want them to be able to install it on one computer. In the distant past we sold software on a floppy that when it was installed on a computer locked itself to that computer If the user tried to install it on other computers it would bring up a message saying the software had been installed on another computer and tell them to contact us for further copies . Today I'd like this to take them to a website where they could buy further copies I would like to be able to distribute the program both as a download and as a CD