Welcome to the iMac firmware/video problem solver page
The most important thing here is the OS X video/firmware problem solver that helps you fix the problems that are caused when inserting any CD with OS X on it in a slot-loading iMac which has out-of-date firmware.
There are also other useful iMac resources - follow the links below.
iMac OS X video/firmware problem solver v2.1 (23 Oct 2004) 99K HTML format, Rev A (3 Jan 2007 - fixed link for printing)
(You may also need these additional instructions, originally added here 21 July 2004) If your slot-loading Mac has reached the stage where it shuts itself down about 10s after the startup chime (symptom S4 in the problem solver) then there is a fix by Bart Denoo and Frederik Vande Rievere at http://frderi.spymac.net/imacresurrected. Thanks to Bart and Frederik for their effort and lateral thinking in coming up with this workaround and making it available. It has probably saved many more iMacs!
Please be patient accessing this page as the server is a bit flaky (in fact I wonder if it is still there). You may have to try again tomorrow.
Alternatively here is an unofficial mirror of the page, taken on 18 September 2004, which may not be completely up-to-date.
If you don't have to hand the requisite floppy cable and bits of wire that this fix needs then check out the original problem solver for further suggestions. Alternatively, any electronics store should be able supply the necessary.
Two final notes:
Some useful iMac links at mrjcd.com
Tray-loading iMac stuff! - click here
Original versions of the problem solver, which contain some additional tips and information (including information about symptom S4 on slot-loaders and about the monitor flyback transformer problems on tray-loading iMacs), and useful links, but also erroneously imply that tray-loading iMacs can suffer from the same firmware problems, are here:
iMac firmware problem solver v1.2a 27 Feb 2004 (MS Word 79 KB)
HTML version of v1.2 23 Feb 2004 (thanks Donna)
PDF version of v1.2 23 Feb 2004 (plus a couple of links fixed) 152 KB (thanks jcd)