I know that Apple Software Update and the iTunes installers use MSI packages under Windows, so there might be something in event log related to any installation failures that may have occurred. Another thing to try is maybe filter for msiexec.exe in Process Monitor during an update operation to see if anything stands out (unfortunately, it may be a haystack there). 

For some reason, MSI logging isn't enabled by default, and while this knowledge base article (http://support.microsoft.com/kb/958041) is probably inapplicable to your issue, method 3 in that article should allow you to enable MSI installer logging in Win7 -- logs from MSI installs should then show up in %TEMP% if I recall correctly.

Sorry for the hard time Big Smile