Is Windows Defender an anti-virus program? If so, should I turn it off as Norton Security suite recommends? If I turn it off, how do I manage my start-up programs?

Thanks for your answer. The Norton Suite came pre-installed on my new Vista notebook. I may have mislead Norton people by characterizing it as an anti-virus program. I will update Windows Defender, based on your response, and then be able to manage my start-up programs.