Because the TCP/IP protocol is a critical component, Windows XP does not allow you to reinstall it via the GUI. You can reset the registry settings via netsh and achieve a similar effect.

In the command prompt, type

netsh int ip reset logfile.txt

More info is available at:

http://support.microsoft.com/kb/317518
http://support.microsoft.com/kb/299357

If you are having strange networking-related problems, this procedure may be of help.

In my particular case, it seems that the Nokia PC Suite and/or Nokia phone drivers will, in some situations, corrupt or incorrectly set some TCP/IP related registry settings that rendered Windows mysteriously incapable of logging in to my 3G carrier's (Smart 3G) internet service. This procedure was able to fix the problem and saved me having to do a complete re-install of Windows.