Cannot enable Internet Connection Sharing

Could someone please tell me what I am missing here?

I have a home network of Windows 2000 and XP boxes set up and I am trying to
enable ICS on my Vista box.

When I view "properties" for my dial-up connection, there is no "sharing"
tab, as described in the help file, and other boxes on the network cannot
connect to the internet through my connection.

The network is otherwise functioning properly with all boxes able to see all
others, and the dial-up connection works fine on the Vista box -- I just
cannot figure out how to share it.

All connections on the network are set to "obtain IP address automatically",
and to "obtain DNS server automatically"

"Discovery and sharing" is set to "allow"

Anyone have any suggestions to fix this?