Before we go ahead,could you post up a screenshot of your disk management?
Just incase you don't accidently wipe the Disk with boot files on it.
*Start>Control Panel>System and Security>Administrative Tools>Computer Management>Disk Management (bottom left).
guide to upload your screenshot after taking it with the snipping tool.
If everything looks good theres two options depending on the situation.
If you have files on D: you want to keep you can simply go computer>D: (double click) > Select the file you want deleted and click delete. Then go to recycle bin and permanently delete them.
If you have no files you wish to keep on D: and would like it fully cleaned you can format it by Right clicking D: in My Computer Page>Format>Format as NTFS/Quick Format>Start.