Level 0: Press F1
Level 1: Press Numlock, scroll lock and caps lock
Level 2: Press backspace to delete the text
Level 3: Type ‘open’.
Level 4: Use the scroll wheel of your mouse.
Level 5: Put the mouse cursor in the middle of the chasm and jump onto it. Now you can jump to the other side as well.
Level 6: Hold your mouse off-screen. It’s now possible to jump from block to block to the other side.
Level 7: Press the Window-Key
Level 8: Insert a CD when drive is empty, Remove CD when not.
Level 9: Set system date to 17/11/1989
Level 10: In the game folder create a text file with the name ‘doors’. No quotes, no extension.
Level 11: Press the Insert-key and the Home-key at the same time.
Level 12: Start the calculator (Start/Run/Calc).
Level 13: Remove HKEY_CURRENT_USER\Software\DOORS from your registry.
NOTE: You can use this command-file to do the work for you if you don’t want to use Regedit. In either case, you change the registry at your own risk!
Level 14: Tricky one. Kill the task doors_vista.exe (or whatever your exe was called.). Do this through Task Manager (CTRL/ALT/DEL or right click on the taskbar). Now restart your game and replay level 14.
Level 15: Wait for the music to end.
Game source: A copy of the game was found here on the internet.