The process for installing updates on Windows is pretty clunky, because MSI doesn't handle minor upgrades very well. InstallShield solves this by creating a zipped executable file containing the MSI file and a little program which automatically uninstalls the previous release if necessary. As you're still producing frequent updates, and your customer base must be quite large by now, how about splashing out on InstallShield (or any other installer with that feature), to make life easier for us?