I deleted game two times from steam reinstalling still wont work im not sure what is causing this i do have fomm installed but i have no mods only have fnv modded all i have is dlc in right load order i tried launch from steam and fomm i get same crash and the game did work find before its been crashing on start since i got my new graphics card. I had a gtx 285 i upgrade to a gtx 580 but i dont know if thats it or what i recognizes the card in the menu fine.

My specs
i7 920
gtx 580
6g ddr3 ram
Windows 7 64bit

Have you deleted the .ini file at any point after you got the new card? Should delete it so that when you start the game a new one gets created that adjusts to your new hardware config. The .ini file is located in your My Documents\My Games\Fallout3 folder...it is the configuration settings file named FALLOUT.

Delete it, then restart the game and a new one will get created.