Microsoft Knowledge Base Article 315669 contains the following summary:

Denial of service attacks are network attacks that are aimed at making a computer or a particular service on a computer unavailable to network users. Denial of service attacks can be difficult to defend against. To help prevent denial of service attacks, you can use one or both of the following methods:

Keep your computer updated with the latest security fixes. Security fixes are located on the following Microsoft Web site:
Harden the Transmission Control Protocol/Internet Protocol (TCP/IP) protocol stack on Windows 2000-based workstations and servers. The default TCP/IP stack configuration is tuned to handle normal intranet traffic. If you connect a computer directly to the Internet, it is recommended that you harden the TCP/IP stack against denial of service attacks.