If you are giving someone a PC it is a good idea to format the hard drive and re-install Windows.
If you dont do that all sorts of information about you can be stored on the machine, even if you think you have deleted everything.
Files you thought you deleted can still be there, passwords for your bank and other organizations can still be there, web sites you have visited can still be there and so on.
Normally if you buy a PC it comes with a restore CD so you can rebuild the PC using that.
If you get the chance during the restore, choose to format the hard drive.