I backed up Windows Xp "security updates" to an external drive. How do I install them after a system restore?

No, what you could've done is slipstreamed those updates into the installation media, but Windows won't automatically recognize the files and install them. Your best bet is to either take your computer to a location where broadband is available, or go to a location where broadband is available and download the latest service pack and update rollups, take those home and install them, and then run Windows Update.