Reseting An Old eMac?

If the C key does not work, start your Mac with that disc already loaded in the optical drive, and hold an Option key while starting the Mac. A "startup device chooser" screen will appear with a choice of icons. Use the eMac's keyboard cursor left / right keys to choose the optical disc and press Return.

This is an old eMac with a CD drive and the OS comes on a CD which I’ve still got.

The easy way it to boot from that disc and choose Disk Utility at the Mac OS X Utilities menu. Erase the internal startup disk. That takes only a minute or so.

You can choose one of the "Secure erase" options but they will take a very, very long time (weeks, perhaps) and don't really convey much more in the way of actually erasing everything to the point data recovery becomes a literal impossibility.

To boot from the CD-ROM disc, press and hold a C key while starting the Mac. Release that key when the optical drive starts making noise.

If the drive and everything else is working properly the Mac will eject the disk when shutting it down. Put it right back in.

You ought to be able to do that when starting it up but the timing has to be right, so that it doesn't default to starting up from its internal drive.

Great! Glad to help.