You may have one physical hard drive, but it is probably split into two logical partitions. This was done when Sony built the laptop and installed the software.
These will probably be called your "C" drive and your "D" drive.
My guess is you have been using your C drive and that is full and your D drive is empty.
There is no problem with having a split hard drive, you just now need to start using your D drive.
So when you want to store things just store them on D instead of C.
Or you could move some files from your C drive to your D drive. For example if you have downloaded a movie from the internet and it is a large file you can move it from C to D.
There are utilities around to "merge" partitions, like Partition Magic shown here
http://shop.symantecstore.com/store/symnahho/e n_US/DisplayProductDetailsPage/ThemeID.106300/ productID.44316200
But note if you make a mistake while trying to merge partitions your could accidently delete all the partitions and lose everything on your disk.
I would just leave things as they are and just start using your D drive.