i cannot believe the stupid answers on this question for what is really simple answer for most average computer geeks.
smashing the hard drive into tiny itty bitty pieces? are you freaking kidding me? the man said he wants to give his computer away because he wants to be nice, perfectly reasonable, sheesh.
and buying computer software to just to erase a hard disk once? are you kidding right?
to explain why just doing "format C:" or right-click, format will not be enough to erase the hard drive - when data is stored on a hard drive, as well as the actual data being stored on sectors of the hard drive, there is another section of the hard drive called the File Allocation Table, or FAT. This FAT keeps a record of all the files and the location of files on the hard drive. on a brand new hard drive the FAT and the rest of the hard drive is obviously empty.
as the hard drive gets filled up with data, be it the Windows operating system, MP3 files, porno, college work, Firefox cookies or whatever shiz it happens to be, data is written where there is empty space on the drive, then the FAT is updated so the operating system knows where to find those files for future retrieval.
If you think about the old days in the public library, you used to have a card index with a card for every book in the library, and each book would have a code telling you which shelf to find the book. this pretty much the way FAT works. if you want to know whether you have a particular book in the library you obviously don't look through the whole freaking library on every shelf, you skim through the card index, and if the book doesn't have a card in the index, that book is not in the library.
say you then delete a file on your hard drive. for example you get a girlfriend and you think you should probably delete all the lesbian MPEGs from your computer before she finds out... what is common in most operating systems is that to delete those MPEGs or whatever, you just delete the appropriate entry in the FAT.
now with the FAT updated, as far as the operating system knows, the lesbo MPEGs aren't there anymore, and the hard drive can carry on with its normal business.
the key concept here is that even though the FAT has been updated to say certain files are not there, the actual 700 megabyte file or whatever it's located on a different section of the hard drive will still physically exist.
when you do a format of the hard drive, typically if the drive has been used before, all you are doing is just erasing the whole FAT. however the actual data will still be on the hard drive.
if you go back to the library card analogy, it's a bit like taking the card index and burning it and then putting new blank cards instead. but you've only just deleted the old card index. what about the books? shiz they are still on the shelves! we could re-index the books (it would take ages to go through every shelf but it could be done) and we will have a fully 100% accurate index again.
and this is what you can do with a hard drive is after quick formatting the drive, is that the FAT could be built again and the data retrieved and all the shiz that was on it is fully visible again.
to fully delete the data, as already mentioned you need a program to do a sector-by-sector rewrite of every section on the hard drive, effectively emptying the the books off the library shelves.
there have already been a few high profile cases where council office computers have not been erased properly, so they have just got sold to the general public as second hand and some geeks have ran recovery software to see what may have been on the drives and all private shiz like names, addresses, birthdays and private stuff is all visible again.
there are lots of free software available to do these erases so i wouldn't buy it.
sorry if i waffled on i was soo annoyed at the blantant crap that some people put on here, it's not even sarcastic which i could semi-forgive.