The only way to get rid of previous data on the hard drive is to "wipe" the drive with software that over-writes all sectors with "0" or some random meaningless data.
If you are donating your computer, are you also giving the original factory-software discs too, so the recipient can reinstall the factory software if they have a problem with things like a malware infestation?
If you do have some kind of factory-software discs, then you can "wipe" the hard drive and it is bye-bye to whatever data was on the hard drive. Then you can use the factory-software discs to reinstall the software and the next user will have a computer ready-to-use like it was just out of the box, assuming the hardware has not changed in the meantime.
If you don't have the factory-software discs, find out how to create them (if the computer is able to) or obtain them. Otherwise the only people who will want your computer are Linux people. Ubuntu is getting very popular with some people and they might like having a computer to work with it on.
You can get a free disk-wiping program from
http://www.killdisk.com/
at this link:
http://www.killdisk.com/downloadfree.htm
I got this version :
"Download Bootable ISO Image of Active@ KillDisk to burn CD"
because I could make a bootable CD with it and just pop it into the CD/DVD drive and reboot and select the option to wipe the entire drive.
The site also has some generic instructions here:
How to burn CD from ISO image file ?
http://www.ntfs.com/iso-burning.htm
Most CD/DVD burners these days come with software that can burn ISO images My burner came with Nero and it has that ability.
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The killdisk program on the CD also has the ability to erase only the unallocated space on the hard drive, that is, it erases what used to be the deleted files' data from the surface of the hard drive without erasing the existing files from the hard drive. I've tried it twice and it seems to work (I used a file-recovery program called Recuva to see if it left anything interesting behind for me to recover). That's an option for you to consider if you use the killdisk program, but it's not easy to find on the menu.
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