Game download permanent cookie?

I'm not sure if this is the right place to post but:
When you download a demo version with a limited time (1 hour or 30 days) usage, then you uninstall and remove all relevant floders, how does that software know (keep track) of your computer's ID that when you donload and re-install the program it knows it's not your first time?

 

Could be many different ways, registry stored keys seems to be popular, some rely on left behind folders/files. Hard to say for sure.

 

reggie

So if you dug through the keys you should be able to identify it or is it easier to use a good reg clean program (don't know which).