This came up for me too... apparently for some silly reason Google Earth's uninstall files are in the TEMP directory and, when you manually clean the temp directory like I do (as it's a TEMP directory) the old version cannot uninstall itself so the new one can install. It can't find the directory:
C:\DOCUME~1\Axia\LOCALS~1\Temp\{72E2FA6C-F171-4866-AD14-157D8C9C7EE2}\
Of course it can't, I probably removed it months ago. Unfortunately I also cannot re-install the original version either. Google, GET WITH IT. YOU DO NOT LEAVE THE UNINSTALLER IN THE TEMP FOLDER!
In regedit (or regedit32 on 64-bit) go to:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Curr entVersion\Uninstall
Find Google Earth (mine was {1E04F83B-2AB9-4301-9EF7-E86307F79C72}). It won't simply be named Google Earth, so look at the install path. Remove it.
You will also need to find EVERY instance of "google" in your registry related to Google Earth. It leaves a MASSIVE footprint (well over 200 entries). Remove them ALL (the directory, which will often have a long and weird numerical name, not just the key inside). Most are in
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Curr entVersion\Installer\UserData\S-1-5-18\Components

It took me WELL OVER 30 MINUTES to remove all of this junk BUT once I did, I was able to install it! FINALLY after trying to update for over a year I got this fixed.