Hollie,
I think I know the answer. It shows I've listened to what's been said as this is my first answer

(I'll probably be shot down in flames by somebody who
really knows but...)
I believe you have connected you Cam to your PC using USB. This is fine for stills but won't transfer video. You will need to get a fire wire 6pin to 4pin. You can pick these up for between £5-£100 depending on where you look and what quality you want. Apperently, for the stuff we do, the bargin bucket cables will do. One thing though, don't get a cable longer that 2m, as one posting I read reckoned that this is the limit where things can be transferred successfully.
You will also need the appropriate port on your PC.
(How was that lads!!)