OK, Success!

It turns out that the article on Microsoft's Support site at http://support.microsoft.com/kb/2027004 was heading in the right direction. However their instructions to go to the Environment Variables screen and add a new variable, didn't work for me.

What worked for me was going into the Registry Editor, navigating to HKEY_CURRENT_USER/Software/Microsoft/WindowNT/CurrentVersion/AppCompatFlags/Layers. Here, C:\ProgramFiles(x86)MicrosoftOffice\Office14\POWER PNT.EXE was set to DISABLETHEMES DISABLEDWM HIGHDPIAWARE. Deleting the data setting fixed my problem with the file dialog box. Hopefully deleting this setting doesn't affect other applications.