As has been said, it is almost impossible to remove all traces of personal files off a hard disk without removing Windows as well (Windows keeps all sorts of hidden, secret and temporary files, any of which could hold personal details).
Even a reinstall of Windows with a format will not remove all personal data.
As has been said, you need a program like dban that writes "rubbish" over the hard disk hundreds of times to remove all traces of personal data.
p.s. Anyone who sells or gives away a PC without doing this is leaving themselves open for identity theft and so on.