I have windows 8 and it had a trial of McAfee that ran out. I completely uninstalled it. I went to services and tried to turn on windows defender to automatic and click start and constantly get the following error message. "Windows Defender aborts with the following error message: The Windows Defender service on Local Computer started and then stopped. Some services stop automatically if they are not in use by other services or programs." The start option is Manual and will not retain the Automatically option once I try to start the service; it defaults back to Manual. I have restarted the computer and double checked to make sure the McAfee was completely uninstalled and it is.

Have you ran McAfee removal tool or normal unistallation. Go to this Wiki for getting removal tool: http://experts.windows.com/w/experts_wiki/156.aspx Also, you should check this Wiki: http://experts.windows.com/w/experts_wiki/163.aspx