The first time I ran this game, it booted fine: I played for several hours.

Now- even though I've re-installed it, I just get a black screen with the sound effect of the Eidos logo stuttering off and on. I've also tried running it in many different resolutions, and with compatibility mode, but it's always the same result.

I'm currently running in windows 7


Edit: trying to run the game with this result seems to be doing odd things to my desktop; like moving icons around
or changing the wallpaper size relative to the screen: it's like the game is hunting for the right resolution but is making my desktop resolution change as well. Very wierd.

Did you try disabling desktop composition?

I did based on your suggestion. First time it seemed like the game would start but hung on a black screen.
After that it went back to the same black screen and stuttering intro sound.

I wonder if something in my saved games is doing it, since it ran fine before i had saved anything

I don't think, I feel that the problem is related to the OS and video card. Anyway try running again the first setup: you have to add the "-setup" parameter to the game's executable.

1) Windows key + R (or Start -> Run)
2) C:\Defiance's installation directory\defiance.exe -setup

Choose a "safe" value for resolution: something like 1024x768x32 or try matching your usual screen resolution. Keep trying with different values but still use the Windows Xp SP3 compatibility mode, run the game as administrator and keep the desktop composition switched off.