Unidentified network - driving me mad

On my wired home LAN, I'm building a Vista Media Centre box based on
Ultimate. The mobo is an Asus M2NPV-VM with the NForce 430 chipset (with
onboard Gb LAN).

If I perform a fresh install of Vista (and I've now done this many times), I
select the location as 'Home', which should setup the networking zone to be
'private' as the machine is only in a workgroup. Every time I do this
though, Vista insists on setting the zone as 'public' and only enables local
access, with no Internet connectivity, and sets the network description to
'Unidentified'. The ADSL router connecting to the Internet is a Belkin
F5D7633 (UK), which also acts as a DHCP server for the LAN. I've built a
previous Vista Ultimate PC on the LAN which can access the Internet fine,
and get's a valid DHCP address from the Belkin. I've also built another
Vista Media Centre based on an Asus A8N-VM CSM motherboard, which has an
identical chipset and NIC as the M2NPV-VM, and this also worked fine, so I
know the router is not at fault. If I look at the network map, I can see the
local host, connected to the LAN, but then a red 'X' to the Internet. If I
diagnose the 'X', and try and let Vista do it's thing, it just says there is
a problem that can't be fixed.

I've rebuilt this machine loads of times now, trying various BIOS settings,
but every time, Vista sets the location to 'public' and restricts Internet
access. I can't even get a DHCP address on this setup, and Vista sets an
APIPA address instead. If I force the mode to 'private', and set a static
IP, I can ping all other LAN devices, but get no response from the Belkin
router! Vista can see other LAN clients ok, but still won't access the
Internet even with the correct gateway and DNS set (the Belkin). I even
tried an external PCI NIC just in case it was an onboard NIC fault, but this
does exactly the same thing! I disabled IPv6, ran all tests I could,
ipconfig output looks fine.

Obviously this install of Vista is detecting something on the LAN which it's
not happy with, but this 'zoning' of access to network resources is an
absolute PITA. Can this be disabled in any way? Is there a non-essential
service that can be disabled to stop this happening. Why, when other Vista
installs on the same LAN work ok, is this one causing me so many problems?

Before something get's thrown out the window, does anyone have any ideas?