No matter what I tried (deleteing folders, profiles, mozreg files, registry keys), running the Firefox (1.0PR) or Thunderbird (0.8) installers would always result in text that said it was trying to install Thunderbird 0.6 (which looks positively bizarre w/ the Firefox logo. Turns out it was a stale tmp folder in Local SettingsTemp.
Now that was a frickin annoying bug. (ns_temp is evil)