I haven't a windows machine to run a sniffer on to figure this out myself, so I'm here.
When Vista or 7 connect, they then do something and report either no connectivity, local only, local network only or connected to internet. (paraphrased)
Where do they connect? Might it be a simple DNS request? or are they pinging Microsoft.com or such?
My friends won't give up their computers for a short bit while I test this... but that's ok, I do maintenance for them, so one'll be here in a few weeks... it is windows.
