I am having a similar issue, the RegisterExtension() fails and returns a 0x80040154 error.

private bool Register(bool install) { GoogleDesktopSearchRegisterClass gdsReg = null; GoogleDesktopSearchAPILib.IGoogleDesktopSearchComponentRegistration gdsRegistration = null; gdsReg = new GoogleDesktopSearchRegisterClass(); object [] componentDesc = new object[6] {"Title", "Title here", "Description", "Descr here", "Icon", "no icon"}; gdsRegistration = gdsReg.RegisterComponent(kComponentGuid, componentDesc); gdsRegistration.RegisterExtension("wmf"); return true;