Use Inno Setup to delete another application not installed with Inno Setup during installation

I have an old VB6 application that is deployed using开发者_StackOverflow中文版 a very old 16 bit version of InstallShield. I have created a new installer with Inno Setup to be 64 bit friendly for my VB6 application with updates.

All of my users already have the existing InstallShield version installed. Currently, I have Inno Setup working. The problem is, I do not want to overwrite the old InstallShield installation files, so my Inno Setup installs to another directory. This causes problems because now I have two versions installed, with two separate icons.

I need a way to invoke the uninstaller of the old InstallShield application when I install the new Inno Setup version. How do I call the uninstaller from the Windows Add/remove programs through Inno Setup? If i reuse the app id for the InstallShield version to be the same as the Inno Setup, and then manually delete the old files, would that work?

You would have to locate the InstallShield uninstall registry key which will reside under either HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\ or HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\ and then execute its UninstallString value. Once that is done, delete the registry key: it is not possible to reuse the same key with Inno Setup since it will always append _is1 to your AppId to generate the uninstall registry key name. Additionally on older Windows versions it may also truncate this key name and append a checksum, because of length limitations.

Do all this from a PrepareToInstall event function.