Hi,
This can be done by using an "Error Message" custom action.
Here are the steps:
- in the "Custom actions" page, under "InstallUISequence", you show the "Begin" standard action (you can use the "Show Standard Action" button on the toolbar)
- for the custom action you set the "Message Text" field to "The product is already installed!" (for example)
- in the "Expression" field you add this condition:
You can read more about custom actions here:
http://www.advancedinstaller.com/user-g ... -page.html
Regards,
Cosmin