The power surge that occurs in a lightning storm is not always what kills an older Mac. Sometimes it is merely confused (not killed) by the power outage that follows. Because the PRAM Backup battery may be old, it may not be strong enough to maintain the parameters and keep the Power Management Unit working properly.
Consider replacing (or removing temporarily) the backup battery, and afterward, while it is open, press the tiny button labeled CUDA Reset on the motherboard. This diagram will show you the location of those components:
http://www.macgurus.com/products/motherboards/mbppcimacorig.php
The best (non-nuclear) way to thoroughly erase the data on your hard drive is to initialize with the "Write Zeroes" or "Zero all Data" option. This removes almost all traces of your data, and only the US National Security Agency (NSA) could get back part of it if they invested huge amounts of time, energy and lab equipment. If removed from your iMac, that drive could be installed in a desktop Mac or an external enclosure to be Zeroed. There are also "non-enclosures" for as little as US$20 that can power the drive and supply a USB interface so that it can be zeroed or re-used.
A working iMac can still be used by some not-for-profits. Many schools prefer Macs that are 10.5 Leopard-capable. (G4 or better.)
Message was edited by: Grant Bennet-Alder