When you connect your HV30 to the computer via the firewire cable, and turn it on, does Windows recognize it? If not, you may have a bad firewire port, either on the computer or the HV30. Very common.
Using Vegas Pro 8.0 with an HV30, when I click Capture I get a popup window with a couple of options, one of which is HDV. (Can't remember the other one.) As David said, make sure you choose internal capture.