Okay, here we go. We have uploaded a demo for you:
http://www.lindersoft.com/projects/LockedFolder.zip
Please use your own Product GUID in Line 37. This will read the installation folder from a previously installed product. If the installer detects the installation, it sets the runtime variable %VAR_ALREADY_INSTALLED% to "1". In Line 58, we set the Dialog Label to DLG_SELECTFOLDER.
In Lines 62-64, if %VAR_ALREADY_INSTALLED% equals "1" (means: the product is already there), skip (hide) the Select Installation Folder dialog.
There are quite a few more ways to handle this. But does this one work for you?
Friedrich