You can install DLL files used only by a single application in the same directory as the application. DLLs that will be used by a number of applications should be installed in a location accessible to all of those applications. A common convention for locating such community DLLs is to place them either in the Windows or the Windows\System directory. A better way is to create a dedicated directory for the common .DLL files, similar to the way the Borland Database Engine is installed.