Dark Souls : Prepare To Die Edition won't start up

Probably related to GFWL

Make sure it's installed and let it update before you run the game.

I wish they made a version without GFWL now that there isn't any online content anymore for PC users.

I'd guess a GFWL problem also, but you can also get a crash by using dsfix + in-game anti-aliasing. There's probably a better troubleshooting guide in the steam forum somewhere, but offhand I'd do "verify integrity of game cache" in steam, see if the GFWL thing and the Windows Live ID Sign-in Assistant thing are both working properly by trying to run that "games for windows marketplace" program, and if that works temporarily disable the dsfix mod. Good luck!

As people have said, its probably GfWL, as it breaks all the time. You can reinstall it from here: CLICKY