If you're sure you don't want any of the files, then you do the following.
Insert the 'Install & Restore' disc which came with your computer (if that's your copy of Panther) and follow the instructions. It will shut down your computer and boot from the disc - alternatively, put the disc in, shut down and hold down the letter C while you turn the Mac back on.
You have the option of keeping your application and user files or a completely clean install. It's obvious which options to choose.
It does take some time to do it so be prepared to leave it running for an hour or so. The main nuisance is having to then download all the updates that there have been since your restore disc was burned!