Many thanks vitamin for your reply.
The trace you suggested didn't do the trick but the bug link you gave led me to try WINEDEBUG=trace+msiexec and that did do the trick.
My case is not bug 9628 as I get:
trace:msiexec:main argvW[1] = L"F:\\WPOX3\\ISUS.msi"
trace:msiexec:main argvW[2] = L"APP_PRODUCTCODE={83FBD495-DDF6-4C8D-92D6-10261DD6F6A3}"
trace:msiexec:main argvW[3] = L"APP_DWUSLANG=1033"
trace:msiexec:main argvW[4] = L"APP_DWUSINTERVAL=30"
trace:msiexec:main argvW[5] = L"APP_VERSION=13.0"
trace:msiexec:main argvW[6] = L"NESTED=1"
trace:msiexec:main argvW[7] = L"/qn"
which I believe is a case of:
> NOTE: Product code on commandline unimplemented as of yet
This is serendipitous as the installer that won't install is Install Shield Update Service, which you really really don't want as anyone who has ever had anything to do with this in the real Windows world will confirm.
