Review the troubleshooting tips in these articles:

"Software Update Incomplete, This Windows Update Software
Did Not Update Successfully" Error Message When You Visit
the Windows Update Web Site
http://support.microsoft.com/default.aspx?scid=kb;en-us;319585

Windows Update Troubleshooter
http://v4.windowsupdate.microsoft.com/troubleshoot/

Problems When Viewing or Downloading From Windows Update
http://support.microsoft.com/?kbid3385

HOW TO: Troubleshoot Problems Connecting to Windows Update
http://support.microsoft.com/default.aspx?scid=kb;en-us;818018

How To Fix The Cryptographic Service Error
http://www.updatexp.com/cryptographic-service.html

Also, visit  http://www.kellys-korner-xp.com/top10faqs.htm and scroll
down to "Why Can't I Download Updates" for additional troubleshooting ideas.

Visit the Windows Update newsgroup and read the messages
posted there for helpful troubleshooting tips.

The "Official" Windows Update newsgroup:
news://msnews.microsoft.com/microsoft.public.windowsupdate