I just upgraded to Windows 8 and installed Fallout and the patches suggested in the Sticky Fallout MODs thread. It actually works fine, but every time I start the game it complains there was an error with file d3dx9_31.dll.

So it is really just a minor annoyance, having to click past this error message every time the game starts. But any suggestions as to how I can get rid of it ?

Nevermind. It just turns out I had forgotten to install the DirectX libraries. I thought they came with Windows Update.

If anyone else should bump into this problem, the installer can be found here:
http://www.microsoft.com/en-us/download/details.aspx?id=35

This actually helped me.

Incredible. Easiest and most effective fix I could find.
The resolution was so bad that I couldn't see a third of the screen, now I can finally use the high res mod.
Thank you so much!