IIS 5.0 – URLs Require WWW

The company I work for is using Windows 2000 Adv. Server for their web server. With IIS 5.0, all of our websites require WWW. infront of the URL

http://www.domain.com works fine, but http://domain.com returns an “Under Construction” page. I was wondering if anyone knew how to go about resolving this. I would like to make it so that the WWW is not required in the URL.