The answer depends on how much work you want to put into it. There are weaknesses to any system you use.
You can use a file written to the disk as an effective key, if you encrypt it well enough someone can't crack it. I have used a system in the past that got the user hard drive serial number and date. It put them into a file the first time. It would also record the number of times it was executed every time, along with the last date it ran in the file, and then encrypt the file. You can run a simple test to check "today's date" against the last time it ran, if "todays date" is earlier than the the last date it ran, the person has tried to set the PC clock back, so you should them disable the execution.
I have also used registry entries, Labview has the ab
ility to read & write to parts of the registry, you can use this to put a key in there to do essentially the same stuff, but be aware that the smart users may try to tinker with or remove the key, just like they would with a file.
Both of the methods above are still vulerable to smart PC users. The most secure (and labor intensive) method I have ever used required each person to email me a file with critical details, and I would record the data in a database, then send them an encrypted key file.
