No, there is no way to stop the nag unless you manage to register it because the developers are not in the business anymore and the application is discontinued. You need to apply a compatibility tweak and then run the software as Administrator. To apply a compatibility tweak, right click it and select Properties > Compatibility. After this procedure, choose an operating system (eg. Windows XP Service Pack 3) and accept the changes. Relaunch the software and enter the registration key. It should work this way.