Microsoft has released a terrific new white paper:

This white paper provides information about the communication that flows between components in Windows XP Professional Service Pack 1 (SP1) and sites on the Internet, and how to limit, control, or prevent that communication in an organization with many users.

In other words, this paper debunks the FUD surrounding XP's communications with the Internet by explaining when XP connects, why, and what it sends or receives. Highly recommended.