I played the game(no full screen) once for about 90 minutes. I changed one setting if I remember correctly. I chose not to add my scores to leader board. No problems. Close game and try to reopen it later. Crash to desktop with error below.
All three steam to start game fail.
Steam validated game files.
Uninstalled game & ensured no files left behind in keeperRL folder. (Not sure if any there are any in appdata, I couldn't find any)
Run as admin on keeper.exe did a crash to desktop as well but no error message.
Disabled Microsoft Security essentials  no change
Edit: Rebooting Computer did not work either.

A:SteamsteamappscommonKeeperRL>keeper.exe –override_settings FULLSCREEN=n
Error: EXCEPTION_ACCESS_VIOLATION
Error: EXCEPTION_ACCESS_VIOLATION
The process cannot access the file because it is being used by another process.
The process cannot access the file because it is being used by another process.
The process cannot access the file because it is being used by another process.
The process cannot access the file because it is being used by another process.
The process cannot access the file because it is being used by another process.
The process cannot access the file because it is being used by another process.
The process cannot access the file because it is being used by another process.
The process cannot access the file because it is being used by another process.
The process cannot access the file because it is being used by another process.
The process cannot access the file because it is being used by another process.

A:SteamsteamappscommonKeeperRL>if exist stacktrace.out (

mshta “javascript:var sh=new ActiveXObject( 'WScript.Shell' ); sh.Popup( 'Keepe
rRL has crashed 🙁 An imp has been dispatched to report the details to the devel
opers.', 0, 'Sorry!', 64 );close()”
msg * “”
)
  % Total    % Received % Xferd  Average Speed  Time    Time    Time  Current
                                Dload  Upload  Total  Spent    Left  Speed
100  127    0    0  100  127      0    108  0:00:01  0:00:01 –:–:–  108