I see from an earlier post today that you have a new Dell E521. Do not delete this partition or the data in it without understanding the consequences.
The backup drive you see as the d: drive is a partition on your hard drive put there by Dell to contain the Restore data that can be used by you shoulod you get into real problems and need to restore your computer back to its original settings. NOTE This will destroy any data and programs you have loaded or created so it is a feature to be used only in dire emergency.
The restore feature is described in your Dell manual under Dell PC Restore. In this section you will also find information on how to remove the Restore feature should you decide to do this at some stage. As your PC is new I wouldn't recommend you do this.
Its a Dell so the files are there and they are hidden. They do this deliberatly so there is no chance of you going in and deleting them accidently. Can only repeat what the others are saying don't delete it.