>You may try the hard disk serial number. You can use my DriveInformation >class to extract this info (downloadable from the Files section, here on >UT). This is "unique enough", but it will change if the user formats the >drive. > >Another unique number is the serial number of the network card, but I don't >know how to get it (and not all computers have a network card :)). Neither would do. I've seen a scene, a couple of times, where the key was connected to the hard disk serial number, and the OS was upgraded from DOS 3.x to something newer, and it required formatting, or the app was simply moved to another disk when the original disk was too small. The app was not functioning anymore, because the disk number has changed. The programmer who did it was unavailable, and it would be quite undecent if I tried to reproduce what the users had to say. One'd better weld it to user's name (as written on the check), and have it visible at app start. This is quite common practice, and is machine independent.