Harddisk full but nothing on it

try showing hidden folders and files...could be a virus or someone drop a lot of hidden files on the drive.
http://windows.microsoft.com/en-US/windows7/Show-hidden...
try booting off a hirem boot cd and do a virus scan.
also check to make sure a virus or someone has not changed the partion size of your d drive. do about this drive in windows it should still be the same size as when you started.
if not you may have to format the drive to fix the issue..formatting the drive will cause all the data on the drive to go bye bye.

Try to format this drive. Format always can erase everything on the drive.