How do I figure out what is blocking it? I don't have any obvious security software running as I just uninstalled Norton.

If you don't have any third party security software installed, then it is most likely the built in Windows Firewall. You will need to add an exception in windows firewall for the herd protect scanner.

Here is an example on adding an exception to Windows 7 Firewall:

http://www.sevenforums.com/tutorials/542-windows-firewall-add-remove-exception.html


If I were you, I would keep a zero day defense suite such as Comodo Internet Security Premium, along with Herd Protect. Those two together, are the dream team.