Each time Setup starts, it checks the currently active processes to see if the application that it will install or remove is not running. If it were running, the installation or removal would fail because one or more files would be in use.
If Setup detects that the application is active, it displays a message box to ask the customer to close the application before continuing, or to cancel the installation or removal process.
This check is based on the name of the main executable for your project, as set on the Project page or App Paths settings pane. If you do no specify a main executable in your project, the check will not be performed. In addition, the check is not performed on Windows NT4 systems.
Tip - You can disable this check by clearing the Check running processes option on the Installation page.