It looks like either the game itself or one or more of your save files became corrupted.
You can fix the game by verifying its integrity or reinstalling. To verify its integrity, right click the game on the Library screen in Steam, select "Properties", then the "Local files" tab. There will be a "Verify integrity of game files..." button. Or you can uninstall the game (right click, "Uninstall...") then redownload it.
All save files and settings should remain intact, but it's still a good idea to backup them before an uninstall. Their default location is "C:\Users\<your windows user name>\Saved Games\The Quest". The config is called "config.ini" and the saves are "Save0.save" ... "Save20.save" plus "QuickSave.save" (extensions, the part after the dot are not shown usually). You can copy these files anywhere you want.
You can also check which of your save file(s) are corrupt this way. The autosave ("Save0") seems to be okay (that's what gets loaded when you select "Continue"). You can move all other save files to a temporary location and then copy them back one by one and try to load them (you'd have to restart the game between each trial). If the game crashes after you copied back one particular save file, delete it and continue with the next one.
You can also send me your save files and I can check them for you. (To
redshift@redshift.hu)
I also recommend checking your disk for file errors. This is not Quest related, but if you search for "checking disk for errors windows" you should find what you need.