A small amount of users reported having issues with Windows XP, Service Pack 3, and Internet Explorer 7. It seems the site was incorrectly seeing Internet Explorer 7 as version 6 and blocking access to it, as it does normally to all users with Internet Explorer 6. After some research, the best explanation I found was that Service Pack 3 sometimes causes Internet Explorer 7 to use an incorrect user-agent string; which makes it appear to be version 6. For you developers out there, I changed the browser detection for Internet Explorer from jQuery to CSS conditional comments. This seems to work perfectly now.

A big thanks Bob from RJM Computer Consulting Group, Inc. for first reporting the issue and helping confirm the fix.