My game DVD gives used space as 4.7GB and 0 free space. My blank DVD tells me 0 used space and 4.7 GB free space.
Can you read from it? If so, no problem. Can you write to it? If so, no problem. Maybe it's the model of DVD you have, dunno...
Probably something with your drive, you can ask a friend to make a copy of the disc (as long as it's legal) you are trying to read, and the copy should work in your PC.