Run Software as Exclusive Shell Application?
The key in the registry is called:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon -> Shellwhich should be set to explorer.exe. BPM Studio probably changed this key.
This sounds very much like a custom shell.
All the MSDN documentation that I could find seems to concentrate on embedded versions of Windows where replacing the shell is clearly very common. However, I believe that the principles are much the same for desktop Windows.