You could try the freezer trick. Pop the HDD in a freezer bag in the freezer for about an hour, then connect it upto to another machine and try using a zero fill utility.
Not sure what that would do??
The freezer trick is often used to "recover" a duff hard drive, for just long enough to get the data off it. This can work because many electronic failures are temperature sensitive, and cooling causes things to shrink (as most men will know) so if there is e.g. a bad solder joint the cooling will bring it together.
To answer the original Q, if you delete everything you can off the HD, and then write a number of huge files to it, enough to fill it right up, that should do it.
But - assuming you are leaving Windoze running - there are still loads of places where embarrassing stuff could be left. One electrician working at my house a while ago asked me how one can get rid of the file search history menu - he didn't want his wife to see the filenames he was searching for
It turned out to be stored somewhere in the registry.....
Must delete the windoze swapfile of course.
If you are prepared to delete Windoze and just have the laptop booting into say ms-dos 6.2 then it's easy to clean the HD. But then you may as well use Killdisk or something like that.