Ok.
If you sell that PC with the drive in it then you have to assume that anything on the drive could be recovered.
You can get a good level of assurance that the contents of the drive have been deleted by using
Secure Erase (you need follow the instructions on the site to create a bootable floppy or a bootable CD with it on), but in theory even that isn't perfect.
BTW, Secure Erase is used by certain VERY well known LARGE organisations, to scrub some VERY sensitive data!