Microsoft recommends that you use the Office Update site to determine if your computer requires this update before installing it. This update enhances the junk e-mail filter in Outlook to help provide a higher level of protection against junk e-mail. As part of Microsoft's ongoing commitment to reducing junk e-mail, periodic updates will be provided to Outlook users. This update is optional and simply provides a more up-to-date definition of messages that are considered to be junk or not junk.
For more information on this update, see the Microsoft Knowledge Base article (832333) Overview of the Outlook 2003 Junk E-mail Filter Update: KB832333.