You'd think people would be wiser by now, but apparently many are still
carelessly disposing of their PCs full of personal information. Personally I'd always use one of those utilities that overwrites the whole hard drive.
In theory, I suppose extracting the actual disk and abrading it on a linisher might be more secure, but extracting the disk is easier said than done; I tried it once and the darned thing did its very best to stay in place!