After upgrading to servicepack 3 in Windows XP you might get the following symptoms:
After you install Windows XP Service Pack 3 (SP3), Device Manager may not show any devices and Network Connections may not show any network connections.
This problem may occur when an antivirus application is running during the installation of Windows XP SP3.
Microsoft has created a small utility you can download here: http://www.microsoft.com/
More information about this problem can be found in KB953979