Do you, or have EVER, had any hacks, downloads, etc that were not "official"? I'm still going with my theory that there is an issue with the install. When you uninstalled, did you go so far as to delete the game folder that was in the C:\Program Files\Microsoft Games\Zoo Tycoon 2 folder? (This is different that the other folder I told you NOT to delete).
I had an issue with my game because of a download that I tried and tried reinstalling but didn't delete that folder. When I reinstalled the game, still had the problem. So I uninstalled again and made sure I deleted that whole folder. No further problems when I reinstalled it the second time.
If that doesn't work, I'd contact the Microsoft Games Support
http://www.microsoft.com/games/zootycoon/zoo2/support.aspThey live for this game...some of us here have lives. ;-)