This is a very common Problem in Vista. Consumer/Residential Internet Providers are set for Automatic DHCP. The ISP's do not even know the answers and Microsoft support has had problems getting people online. This is with the PC directly connected to the cable Modem. the first step to getting the connection going.
Working for an ISP, I would like to know the real answer for getting Vista online. It is frustating not knowing what truly is the fix. Of course we have a couple Vista PC's set up to explore on, but they are already connected. Especially for those who buy new computers and attempt to set up for the first time. This will become more prevalent as more consumers purchase Vista machine. This will probably happen this 2007 Holiday season & Microsoft comes up with a reasonable Update for most big issues.
I have attempted disabling IPv6, checking settings, disabling & re-enabling network card and LAN connection and still nothing in most cases. I have tried unplugging the power from the computer so that the network card & the system loses information holding up the connection. one time I switched over to USB and that worked. Luckily the person was not setting up with a Router.
I think more reseach needs to be done on this.