I was working on an Outlook add-in that was making use of WPF User Controls hosted in an ElementHost within a Windows Form.. On all of my development and testing machines, the code worked fine, but on one of our notebooks with Windows XP SP3.. the window rendered as a black box only. The window border was present, but the contents were completely blank.

After much trial and error, I found that the culprit was that the TransparencyKey had been set on the windows form that was hosting the ElementHost. Removing the TransparencyKey fixed the problem...!? Now the UserControl is able to render properly.

I'm not quite sure why setting the TransparencyKey showed up as a problem on only one of our machines... driver bug? WPF bug? Anyway.. at least it works now. Hopefully this will be helpful to others if you run into a similar issue.