I have a new in the box dell 630 with vista ultimate.  The only way I can connect to the internet is by enabling the linksys router, wrt54gs, to broadcast it's ssid.  I cna then disable it broadcasting it's ssid.  When I turn off the pc and turn it back on, I again have to enable the router to broadcast it's ssis before I can connect wirelessly.  I have added the PCs mac address to the allowed list of PCs.  I have a dell 620 with vista ultimate that can connect to the internet with no problem.