Re: How do some programs know how long you've used it even after you uninstall?

Great question, Duane! You've probably just had a painful introduction to the Windows Registry, a central database that exists on all machines running Microsoft Windows 95 or later. The Registry holds information about programs, user preferences, and so forth. Most likely, the information about your software is hidden somewhere in the Registry, and that's why it doesn't go away even if you delete the application.

I myself am not particularly fond of the Registry. My main objection is that, since so much critical informations is stored in one (hidden) file, then if the Registry becomes corrupted, there's not much you can do but restore it from backup or, failing that, reinstall your entire operating system. It also makes it very hard to move programs around from one disk partition to another. Say you've got a bunch of programs on your C: drive, which has become full. Under Windows, you probably can't just move them to the D: drive because the Registry knows they're on the C: drive, and they may not work after you move them. Fortunately, there are some programs that help you do that. They work by scanning the Registry and updating all the pathnames.

Another (and probably better) way to do time-limited licensing is for the software developer to create an encrypted license key that contains the cut-off date. So when you download a product, you also get an email containing something like LICENSE=XGALKF87148DKJAIO093. So you type in that license to the software, which stores it in a file (still encrypted), and, when the software runs, it decrypts the key and finds that it's good through December 31, 2002, or something like that. This method, of course, can work on any operating system, not just Windows.

Hope this helps!

Best wishes,

Aaron Endelman