I have 2 options in mind, first:
Use Alt+Enter to force it full screen, shut it down with Alt+F4, then relaunch it.
The second idea would be to delete some lines in the register:
Open "Regedit" (In the windows menu, write it down, it should easily find it) Go to "\HKEY_CURRENT_USER\SOFTWARE\GrizzlyGamerStudio\Robin Morningwood Adventure" and delete everything inside. Launch the game again.