I have a weird problem. On my D drive there are 700 and something GB in total, however, it showes only 1.96Mb of free space, what is more weird is that the total files (including the hidden ones) are only 62Gb.
If there's a system in there, what the size of the folder %systemroot%windows\winsxs folder? You may find Googling for and installing a little free utility called SpaceMonger useful here. If holdingholder is right, try running Checkdisk with the two boxes ticked to find and fix errors.