1. A method of determining a user demographic profile of an anonymous user that visits one or more websites of a predetermined group of websites, the method comprising: determining whether an anonymous user's computer has user identification data installed thereon, said determining performed upon the anonymous user visiting a website of the predetermined group of websites, the user identification data unique to the anonymous user; installing the user identification data on the anonymous user's computer if the anonymous user's computer does not have the user identification data installed thereon; generating a first record of visits by the anonymous user to the websites of the predetermined group of websites; providing one or more website profiles comprising demographic characteristics of each website visited by the anonymous user of the predetermined group of websites; and calculating an estimated user demographic profile of the anonymous user by using the first record of visits by the anonymous user and the one or more website profiles.