Millions of computers have updated without a flaw. That means that there is either a flaw in your hardware, operating system or security product(s). Or that there is a bug in the update process that only shows up under circumstances that are too rare to $chase$ down.
While there is always room for improvement (don't tell my wife that I said that) I doubt that you want to spend the hours that it would require for Mozy to ferret out what precisely what went wrong on your "one in a million" computer.
There are 3 things that I can think of for you to try:
1) Use a tool like CCleaner to remove temp files from your system
then manually try a backup
after the backup, the Mozy app should try the update again
2) if the update did not take - then manually uninstall / restart / re-install:
Before you uninstall - place a copy of this file
C:\Program Files\MozyHome\Config\conf.dat
on your desktop. (The path to that file may vary.)
After you re-install, copy that config.dat file back where it goes before you let the fresh install of the Mozy app start/complete a backup.
You can download the Mozy software using the info in my signature.
3) If the uninstall fails (and it might, given the whole MSI thing that you pointed out) See this post:
http://community.mozy.com/t5/Mozy-for-Windows/Cannot-uninstal-old-version-of-mozyhome/m-p/37113/high...
Sig::: Practice restoring a file before you need a file in an emergency.
Using MozyHome (free) 2.20.2.276 on W7 Pro
http://mozy.com/home/download (file and release notes)
http://mozy.com/pro/download (file and release notes)