trinityadder

Hello.. I've got a question about trial programs, and I really hope someone can answer this.

You know those programs that have free 30-day trials and stuff? And then when you try to redownload and reinstall it says this program has already been installed on this computer and the trial has expired, so download the full version, etc. etc. ?

What I want to know, is how does the program KNOW that it has been previously installed? Does it leave a registry entry? Or a file lying around somewhere? It's a technical question, but if you know, then pleeeaase help!

Thanks!

PS: Knowing how a program does it is not illegal.