Fallout starts up, and then freezes at a black screen, until I hit enter or space, at which point it goes back to my desktop and freezes til I close it manually by ending the process (it says the task isn't responding).

I am not sure, but you could always uninstall the game and reinstall it to see if that helps.

I downgraded to my laptop running xp, and i'm getting the same error; the installer says it has full integrity, does anyone know what causes this?