I assume you don't have a macintosh install disk, which woiuld allow you do do a deep wipe. and reformat. I also assume that the thing boots.
If it boots, take a look to see what apps might be on it. If you get lucky, you'll find Norton Utilities. In the Norton Tools folder you'd find Wipe Info - designed to overwrite everything.
One of the easiest ways to get rid of info is to overite everything. If you open the harddrive, you can trash everything but the System Folder (OS9 and earlier) and empty the trash. If you have access to a graphics app on any pcs (like a photoshop), grab some stupid but big image off the web and open it in the graphics app (NOT Word). Change the resolution to 600 - 1200 dpi - resulting in a HUGE file. Put that on a CD and put it into the mac. Make a folder on the desktop and copy the pic to that folder. Now just keep duplicating (File menu/Duplicate or Command (Apple key) D) the thing until the drive is full.
To be doubly sure, you can then trash everything except the original, empty trash, and do the duplication thing again. You'll now have 2 overwrites over the empty parts of the drive - very hard for anybody to ever recover anything. That's basically what Norton Wipe Info does.
Or you could just remove the hard drive from the machine.