Do a clean install. And choose to format the disk with 8 way random overwrite (or whatever it's called) - I cannot remember if this is an option after you've chosen install type or if you have to run disk utility from the tools menu in the installer, but you probably find it... 😉
This erases EVERYTHING form the disk before you install Panther or Tiger again, and leave no (or really very little, but don't get concerned over it) trace of anything you had on it.