Welcome to the Forums Claire. There's a couple of possibilities here,
1. Check your capture settings. In the Capture window click the 'more' button. Select the Capture options and ensure that the Capture Format is set to DV/IEEE 1394 Capture
2. Perhaps your Firewire Card is not seated correctly or has a conflict with another card. Try the card in another PCI slot. Alternatively the Firewire cable may be damaged, so you could always try another cable. More rarely (and it is rare) there could be a problem with the camera's firewire port. But try the other options first. Also if you have other capture software (eg Windows Movie Maker) see if you can capture from that.
COMPAQ something or other with 500Gb Boot Disk, 1Tb external Firewire Disk, 4Gb Memory and a super duper sound card, 19" TFT Monitor etc etc etc
Sony TRV33e : Sony TRV310e : Canon XM2
Sony Vegas Studio editions to v11
Remember, there is always more to learn than there is to teach.