You have removed some program or device that had files called for in the registry.Whatever was removed,probably incorrectly or a corrupted file for something not being used, it is not required for the system to function correctly (apparently). In the absence of knowledge of what it is specifically, a reformat would be the best and quickest way to remove the error message, but if things are working correctly and you don't want to reinstall Windows,it is just a nuisance.