From: SANS - Internet Storm Center - Cooperative Cyber Threat Monitor And Alert System - Current Infosec News and Analysis, a report that finds that an unpatched Windows XP box has on average 20 minutes before being infected by a port-scanning worm. They have an in-depth pdf showing how to connect a brand new Windows XP box to a Network which details all the steps you need to go through to secure the out-of-the-box network settings before you connect to Windows Update. No mention that I could see about how being behind a dsl/cable modem sharing router affected survival time.