If you can't aquire any video from a DV camera connected via Firewire/IEEE1394, check if you have these symptoms:
- Windows Live Movie Maker says "The device is already in use."
- other programs (e.g. NeroVision) show a black box but let you control the camera
The reason seems to be a faulty IEEE1394 driver in Windows 7 64-bit. Check the Device Manager whether you have the Texas Instruments 1394 Controller installed. If so, right-click and choose Update driver…, search your computer for drivers, install manually from a list and select the OHCI-compliant 1394 Host controller (Legacy) driver.
After it is installed, you should be able to capture video again.