Do a google on "relevant search terms like "secure erase disk wipe delete" etc. - add freeware to the search too
That means that you can totally erase the disk to a state that can't realistically be recovered. Note that these programs will usually a) have to run several times to ensure complete and unrecoverable deletion and b) take a long time.
You will then need to install an operating system from scratch, so ensure that you have the relevant installation media to hand, with appropriate boot floppy with fdisk, format, CD drivers etc.
You may be able to selectively and securely erase portions of the existing installation without deleting the entire disk, but frankly I reckon it's easier to blat the whole thing.
Interestingly, for secure deletion of data on hard disks on a project for the MoD, I had to use a screwdriver and a hammer. And no, you can't re-use the HD afterwards
SD