How to boot directly into an application

Solution 1:

Note: This will replace the whole Windows shell with your application. There will be no start menu, no task bar and no desktop.

One way to get it back, is to invoke Task Manager with Ctrl+Shift+Escape and start explorer.exe from there.

To permanently reverse this change, just start regedit.exe again and change the Shell key back to explorer.exe.

How to fully replace the Windows shell with another application

You can change the registry key

HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\Shell

to point to your application. Then it will be started instead of explorer.exe as the shell.