How to fix Microsoft Security Essentials install error 0x80070643

I was working on a client's computer today that exhibited the following symptoms: Windows Updates not installing during each shutdown and could not install Microsoft Security Essentials. MSE installer would show error 0x80070643.

Things to check first:

• Make sure Windows Installer service is running. Set it to manual or automatic (doesn't seem to matter from what I can tell). Click Start if it is not running.
• Check the MSE installer log. Do a search for "return value 3".
  If found, look at the previous lines and see if they give you a clue as to the issue. In my case it said "Windows Installer error 1716". So that lead me to the following fix.

1. Start -> Run -> Regedit
2. Navigate to HKLM \ System \ CurrentControlSet \ services \ msiserver
3. Double left click on WOW64. Change 1 to 0.
4. Close regedit and reboot the computer.
5. Try Microsoft Security Essentials installation again.