A: 

Gotta start with the basics... Have you eliminated the possibility of a run-time crash before the main form is shown?

regentgal
The executable runs fine.
Stewbob
+2  A: 

IT ran some type of Windows update on my machine and that has apparently fixed the issue. It had something to do with a System.Runtime.InteropServices.COMException and an error in a 'Hosting' .dll. I'm not sure why a corrupt Hosting dll had an impact on a winforms application, but it has fixed the issue.

Stewbob
Strange things can and all too often do happen. We don't always understand why, but it is good to be back up and running again. :)
Bill W
+2  A: 

I had a similar issue in the past. The project would not run in debug mode.

It was also caused by a corrupted dll, but not the 'Hosting' one. It's been a long time, so I don't recall the exact dll, but it had something to do with Messaging.

GSTD