相关链接： MSDN http://msdn.microsoft.com/en-us/library/ms790156.aspx

顾名思义，Co-installer是个微软的Win32 DLL（动态链接库）用来协助设备安装。

A co-installer is a Microsoft Win32 DLL that assists in device
installation. Co-installers are called by SetupAPI as “helpers” for
Class Installers. For example, a vendor can provide a co-installer to
write device-specific information to the registry that cannot be
handled by the INF file.

The shaded boxes represent the components that you can provide for the system-supplied device setup classes.
The operating system supplies the other components. If you create a
custom device setup class, you can also supply a class installer.
However, you rarely need to create a new device setup class, because
almost every device can be associated with one of the system-supplied
device setup classes. For more information about Setup components, see Device Installation Overview.

Co-installers can be device-specific or class-specific.