Sometimes the Windows XP TCP/IP protocol stack gets corrupted an you need to reset it back to its original configuration. Back in the old days you use to be able to uninstall and reinstall it. That option is not available anymore.
Now you have to use the NetShell utility to reset the protocol stack. By following the instructions in the Knowledge Base Article you will return the TCP/IP protocol stack back to it's default configuration when it was first installed.
Microsoft even includes their 'Guided Help' feature to help you along the process.