Is your scratch disk full?
You only mentioned one disk, you do have one for the OS and a seperate disk for the AV files don't you?
If you are not sure how much space your disk has you can double click on My Computer, and then right click on the drive and choose Properties. That will show a pie chart that shows the entire disk and how much is used.
Captured video files take a bunch of room. I have almost a Terabyte available and I still run out of room.