Although I agree with ChrisF regarding the value of erasing your data, I've also known too many IT departments that would hand out an old PC without re-imaging it. And in this case, simply deleting folders and zeroing empty space isn't going to do what you want, because Windows applications tend to leave droppings all over the place.
So, assuming that you really want to protect yourself, and aren't prohibited from erasing the hard disk, you should download a bootable Linux install, such as Ubuntu.
At that point, you have several choices. First choice is to simply write zeros over the entire disk, including the partition table:
dd if=/dev/zero of=/dev/sda bs=8192
The only problem with this is that
/dev/sda isn't guaranteed to be the hard disk, and unless you're familiar with Linux, it may not be obvious which device is the hard disk. So it's probably simpler to install Linux on the laptop, replacing whatever's there.
However, that won't guaranteed that someone can't pick up old files from the disk, so you'll need to zero all free space. Once you've installed, boot into Linux, and run the following:
dd if=/dev/zero of=/tmp/ix bs=8192
This is subtly different from what warren suggests: it uses
dd to fill the entire free space with a single file. The
cp command can create what are known as "sparse files," which do not actually consume disk blocks.