I am going to assume your windows installation is on C: and in C: windows
When a DLL is registered, the DllRegisterServer method entry point in your program is invoked. Similarly, DllUnregisterServer is invoked when a DLL is unregistered. As described in this MSDN article: https://msdn.microsoft.com/en-us/library/ms682162%28VS.85%29.aspx
Instructs an in-process server to create its registry entries for all classes supported in this server module. If this function fails, the state of the registry for all its classes is indeterminate.
For COM DLLs, you will need to implement your own DllRegisterServer and DllUnregisterServer entry point methods which do the registering/unregistering as appropriate. Example code for DllRegisterServer can be found here. https://msdn.microsoft.com/en-us/library/dd376682%28VS.85%29.aspx
The end result of registering a DLL is that all of the CLSIDs for the components in the DLL are registered under HKEY_CLASSES_ROOTCLSID. This allows CoCreateInstance to find the correct server when instantiating COM objects from another DLL or application.
DllUnregisterServer will do the reverse, and remove all of the CLSIDs from the registry that were put in there by DllRegisterServer. more can be found here https://msdn.microsoft.com/en-us/library/aa369359.aspx