Beware what you do with your D partition as it almost certainly contains a program to restore your computer back to how it was when you first recieved it as you will not have had an operating system disk supplied with it when you bought it.
Dig out the manual you should have recieved with it and have a good read.
Pamy is right, but I have Vaio laptop and the recovery partition was never shown as a drive. Look in Computer Management > Disk Management and it will show you all the drive structures. You could do as gengiscant suggests, or if the D: is empty and not a recovery drive, just delete the partition.