I have the same problem. Windows Vista Ultimate, VMWare Workstation 7, Avast Pro 5.0.418
If I disable shields while the guest VM acquires IP address, and then enable shields, all works.
Need to know how to make a rule, or will report a bug.
Firewall log reports blocking IGMP with application not recognized (blank) IP 224.0.0.22
Cheers,
--Peter