Unable to erase unused disk space w/windows xp Please Help

I have been unable to successfully erase the unused disk space. I downloaded the version of eraser. I keep on getting the error message "Eraser has encountered a problem and needs to close". The last time it occurred, I was able to get this info re: the error message "EventType:clr20r3 P1: eraser exe P2: 6.09.2620 P3:4fbad9e6 P4: eraser.util P5: 6.09.2620 P6:4fbad9d5 P7: 62 P8:9 P9: system.nullreferenceexception . However, I was unable to right click on the task - the page disappeared. I tried to install the update and that did not work. I am now defragging my computer. Is there a solution to this problem?
Update on my problem... I was able to successfully install the update. I ran the task to erase the unused disk space again and it was not completed due to error. I did copy these sample error messages from the log viewer... "c:\windows\system or Help or system32\ ...did not have its cluster tips erased because it is a system file", "c:\documents and settings\allusersapplicationdata\Norton...the processor cannot access the file because it is being used by another process", c:\hiberfil.sys did not have its cluster tips erased because of error:Access is denied(Exception from HRESULT: 0x80070005CE_AccessDenied) and the same for c:\pagefile.sys.