1a. Check your firewall software as well.
3. Use the task manager to look at the services running to establish others that may be using that same *.DLL file.
Many times the problem is not with the software you are installing, but another application that shares the same file. You may have to stop several other services that are running to install McAfee, then restart anything you have stopped. Typically, another antivirus software (or older McAfee version) is running that uses the DLL and will not release it to be reloaded. DLL files are loaded into memory and not released until a service is stopped or uninstalled. DLL are library files that are shared by many applications. I feel that is the problem.
Sometimes this is a trial and error method of correction. Since the OS will not let you shut off any critical service, (most times) you will be okay.
This was first published in June 2002