Need help - can't open Mathcad 3.1

For some time I have been unable to open Mathcad 3.1.  I twice requested help via the Academic Case Logger, and once via a customer support address, but I have heard nothing.  I am running Windows 8.1, 64byte.

The problem is that all attempts to open 3.1 fail & generate the message: "This application requires ... .NetFramework,Version=v4.0.” Installing 4.0 and its 4.6 “replacement does not remove the message even when all existing copies are deleted.  Also  deleting and reinstalling 3.1 does not work.   Regedit.exe shows "Release REG-DWORD 0x0005c615 (378389 Finally, Prime, Viewable Support & PTC Diagnostic tools. ASP.NET 4.5 is ticked in Programs & Features.

Does anyone know of a fix for this?