There are three ways to register an application DLL with Control Panel:
List the DLL in the [MMCPL] section of the CONTROL.INI file. Use this method when the DLL is part of a system library and handles more than just messages from Control Panel. The following is a sample CONTROL.INI entry:
[MMCPL]
myapplets=mydll.dll
Assign the DLL a .CPL filename extension and install it in the directory that contains the CONTROL.INI file.
Assign the DLL a .CPL filename extension and install it in the Windows SYSTEM directory.