Is this as-shipped from Dell - or a new drive that you cloned?
A 320 billion byte drive is really 298 true gigabytes - it should show more than that. Or are you looking at remaining free space?
Also--that D drive is not a separate drive. It is a partition on the C drive that holds the Recovery files to return the computer to factory settings. Do not use, add to or change anything on the D partition or you will lose the ability to use the factory restore. It takes up 15GB of the 320GB on the C drive and will have about 10GB of free space that it needs to work. To see a diagram of the drive and all the partitions, type Computer Management in the Start box and look at Disk Management.