Some of my users who use Norton (yuck) have been alerting me to a security warning they're getting, something about a "drive-by download" -- I'm not getting any warnings from Windows Security Essentials, nor at quick glance do I notice anything fishy in the file mentioned:

Threat Name: MSIE ADODB.Stream Object File Installation Weakness

Location: http://www.gangsterbb.net/threads/ubbthreads.php

It seems, at the link above, keeping atop Windows Update should fix this issue on the users' side, but what could be causing this to begin with?

I am not sure of the problem with norton.
But I use trendmicro internet security. And I had no issues at your site.