Networking gods not pleased with Vista and SBS on the same network

I have a tough problem that a few folks have not been able to solve.

I have SBS 2003 Premium R2 installed on one PC and Vista Ultimate 64-bit
installed on another. I'm trying to get the Vista PC join the SBS domain so I
can get full file-sharing and network security through AD, etc. The SBS PC
also runs ISA Server 2004 SP3.

I have two NICs each on both PCs. The SBS PC has a NIC built into the
motherboard and another NIC that's plugged into a PCI-Express slot. The Vista
PC has two Gigabit NICs built into the motherboard. The SBS PC has an
Internet connection on the NIC built into the motherboard and I use the
PCI-Express NIC for the LAN. Both SBS NICs have been tested with the Internet
connection and found to be working. The Vista PC has an Internet connection
from a different ISP and both it's NICs have been tested with this Internet
connection and found to be working.

I'm just not able to establish a network connection between these two PCs. I
was first attempting this by connecting the LAN NICs of both PCs to a 100Mbps
Dynex Ethernet switch. This switch and the cables being used have been
successfully tested with two Windows XP PCs and found to be working. When I
connect the SBS PC and the Vista PC to this switch, I get a yellow
exclamation mark in the notification area on the Vista desktop and Network
and Sharing Center says "Limited Connectivity" for the local NIC. SBS has no
such problem, but I was not able to ping either PC from the other.

The way I had this set up, I first had Vista configured to "Obtain and IP
Address Automatically" from SBS. That did not work, so I assigned a static IP
address to Vista in the same subnet as the SBS PC. I was still not able to
ping either PC from the other.

I then followed the advice of some folks on the SBS newsgroup and dumped the
Dynex switch for a D-Link DGS-1005D Gigabit switch. Same problem, I'm not
able to ping either PC from the other. I was talking to Microsoft tech
support on the phone while this was going on and they suggested that I get a
cross-over networking cable, which I did. I plugged it's ends to the Vista PC
and the SBS PC and I still have the same problem. I'm not able to even ping
either PC from the other.

I even uninstalled ISA Server 2004 from the SBS PC. And I also uninstalled
the firewall I was using on the Vista PC. Still no luck.

I don't know how naive this might sound, but does anyone have Vista 64-bit
and SBS 2003 Premium R2 on the same network and functioning correctly?

Thanks.