Reinstalling TortoiseGit after updating to Windows 8.1

This is indicative of, I think, a wider Windows 8.1 problem, but I ran into this pretty serious problem with TortoiseGit when upgrading to 8.1.  TortoiseGit won’t start, so when you try to repair it, you end up with an error dialog.  Worse, it could not be reinstalled, repaired, or even uninstalled!

You get this in the event log:

Product: TortoiseGit 1.8.5.0 (64 bit) -- Error 1723. There is a problem with this Windows Installer package. A DLL required for this install to complete could not be run. Contact your support personnel or package vendor. Action KillCache, entry: TerminateCache, library: C:\WINDOWS\Installer\MSI1CC.tmp

After much trial and error I finally fixed it via:

1. Export your entire registry key via:

   c:\Windows\Installer>regedit /E:A c:\Users\MyName\Desktop\reg.txt

2. Then grep through this big file (about 11 MB in my case) and find every key or value that looks related to the MSI, e.g.,

   [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Folder\shellex\DragDropHandlers\TortoiseGit]
   "ProductName"="TortoiseGit 1.8.5.0 (64 bit)"
   "PackageName"="TortoiseGit-1.8.5.0-64bit.msi"

3. Using regedit, delete all these keys (I found maybe a half dozen).
4. Now you should be able to re-download and install the MSI, overwriting the proper registry settings your deleted, as well as the files.  For good measure, I did this via the command line as an administrator:

   msiexec /i C:\Path\To\Downloaded\TortoiseGit.msi

5. Reboot.

Then I got TortoiseGit working again.  Hope this works for you.