Normally this functionality would come with Internet Explorer 7 but Microsoft decided to give everyone running Windows XP SP2 the ability to test this new anti-phishing tool.
The company will eventually make it available to older Windows systems, too. The tool was built to address scammers who try to trick people into revealing passwords by posing as legitimate banking or e-commerce site. When an unfamiliar site is encountered, users have the option of passing that address to Microsoft to check against a database of known phishing sites. A “red” warning page appears when there's a match.