I am not sure what the problem might be. If you installed MC15 and then uninstalled the older version, this may be the reason and you may try a repair installation or a complete reinstallation.
You also have to ensure that the correct .NET version is installed.
It may also be worth a try to run the command "mathcad /register" from a command windows with admin privileges.
If you search this forum for "Library not registered" you'll find a couple of older threads dealing with the very same problem - maybe the suggestions given there can be of help.
