Which DLL is that? Are you sure other application didn't install it too? Did you remove any install package that you may have installed while developing?
Hi
Are you sure you have Windows Installer 2.0 or higher?
The version of your Windows Installer is the version of file "msi.dll" that is found in winnt\system32 folder. If the major version of this file is not at least 2 then you have to install Windows Installer version 2.0 or higher and test again.