Eraser not erasing



I'm new to this forum and apologize if my question is out of line.

I've tried to use Eraser 5.7 to wipe the unused space on my hard drive using both scheduler and on demand. I've run it many times but seem to get nothing but "Possible Failures". When I use a program to view the unused portion of my hard drive, I find that very little has been removed. What am I doing wrong? My O/S is Windows-XP Home Ed. and my browser is Firefox 1.0.