This has been discussed on here before, try searching.
It is VERY difficult to remove all traces because even a format of the hard drive and a re-install of Windows can still leave traces of your data.
But assuming the person you are giving it to is not a hacker, and does not work for the FBI, then the MINIMUM I would do is re-format the hard drive and re-install Windows.
I all you do is delete a few files then there will still be traces of what you have done left on the computer, which would not be difficult to find.