Microsoft Knowledge Base Article 841823 contains the following introduction:
Microsoft Windows XP Service Pack 2 (SP2) includes Windows Firewall. Windows Firewall is an enhanced version of Internet Connection Firewall (ICF). Windows Firewall is a host-based, stateful, filtering firewall that discards unsolicited incoming traffic through TCP/IP version 4 (IPv4) connections, and through TCP/IP version 6 (IPv6) connections. By default, Windows Firewall is enabled on computers that are running Windows XP SP2.
Because Windows Firewall is enabled, Microsoft SQL Server cannot listen to the network, even if it was previously configured to do this.
This article describes how to manually enable TCP/IP on computers that are running Microsoft Windows XP Service Pack 2 (SP2) for Microsoft SQL Server 7.0, and how to configure Windows Firewall in Windows XP SP2 to enable SQL Server 7.0 to listen for TCP/IP traffic on a static port.