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?

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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.