Well, you have Steam so it's different but I'm sure you do have older saves. Every time you press Save & Exit a new save file is being created. You can find all your save games here:
C:\Documents and Settings\Nik\My Documents/Amnesia/Main/Merry
Copy the above to any window address bar and it will take you automatically. It is based on your HLP.log. I assumed too that you have Windows 2000 because of the attachment in
this reply of yours. But yes, indeed you have Windows XP, sorry about that...
For now we will try older save games. As I told all your save games are in that folder. In order to load a previous save game launch amnesia and in the main menu choose Load Game, not Continue or Start a new game.
Select Load Game and load a previous save game from there. I don't know about Steam though. At google I also found this menu too:
Maybe it is different on Steam? I don't think so. Is this how your game looks like?
If for some crazy reason that fails too, well we will have to do the crazy idea mentioned before as a last hope of you being able to at least finish the game and then really update it from Steam.