Start here: http://social.answers.microsoft.com/Forums/en-US/vistawu/thread/1467f44b-ee27-4f7d-98d7-f1c4b35b3395
How does your problem pertain to Windows Update?
What anti-virus application or security suite is installed and is your subscription current? What anti-spyware applications (other than Defender)? What third-party firewall (if any)?
Has a(another) Norton or McAfee application ever been installed on the computer (e.g., a free-trial version that came preinstalled when you bought it)?
~Robear Dyer (PA Bear) ~ MS MVP (IE, Mail, Security, Windows & Update Services) since 2002 ~ Disclaimer: MS MVPs neither represent nor work for Microsoft