A registry would only work on one machine, the original, installing a registry would not prevent them from installing it on another machine because it would not be present on that machine, a better solution would be to make a custom serial key as was said earlier, for that comp and it only, mabe with its IP address if the computer is never moved or other computer info, talk with the client first, and get the info, or hard code it into the code for custom perchases. Thats just one idea, if you can get the server varification of a one-time-use serial number that would be best, if you MUST stick with registry keys this: http://www.codeproject.com/KB/system/modifyregistry.aspx[^] might help... best of luck!