Every time I try to boot this game I get a failed to start error which hard locks my computer. It worked yesterday why not today?
I have verified the files and tried removing the mp4 files but nothing seems to fix it and I am getting fed up of having to restart my computer.

1 - Change the registry entry in Computer\HKEY_CURRENT_USER\SOFTWARE\Mimimi\Desperados III\Screenmanager Fullscreen mode_h######### (the #s may be different and/or user specific so I didn't include them). Change the value from 0 to 1.
2 - You should be able to launch Desperados 3 now.
3 - ONE MORE STEP!!!! - After successfully launched, go to Video Options and Change Fullscreen mode from Exclusive to Borderless ... if you don't, the above registry value will be reset back to 0 when you exit the game and you won't be able to launch again. Setting it to Borderless will keep the registry value set to 1.

There is clearly a problem with Exclusive mode.

The only other option is to uninstall/reinstall and then make sure the video mode is set to Borderless before exiting.

Rather than reboot ... ALT-CTRL-DELETE ... sign out, then sign back in. Probably a little easier on the computer.

Thanks - why is this not in some FAQ...?

It worked fine in exclusive mode for a while, then I moved from an HDTV to computer monitor, and I promptly got the "Failed to start" lockup. Got back to normal with this answer...