How can one eliminate empty space on a HDD?
Can Defraggler move all files to one part of the drive, to eliminate empty space?
How is it done?
This is desireable so a HDD may be partitioned.
Do you mean filling gaps? If so, the full (regular) defrag will try to do that but it can be a very time consuming operation.
Partition Wizard has many capabilities, including the shrinking of a Partition.
I recommend you download and burn the ISO for their Boot Recovery CD in case Windows gets "bent out of shape"
Lots of good stuff here