It’s an unassigned IGMP address. I would just block it at the firewall. It’s probably just a multicast from your system (host) looking for members.
Here’s a link to IGMP
http://www.freesoft.org/CIE/RFC/1112/18.htm
As a rule of thumb, you should build a list of what the firewall needs to pass and then lock everything else down. In practice, we lock it all down and then open as needed. Often, we apply filters to the PIX’s on a per machine basis. Allowing all internally originating traffic is no longer seen as acceptable.
PS: TM's antivirus has had some bigtime patch blowups in the past.