iMac 24" intel 2009 - will not complete power-up

Have been using my iMac 24" intel (2009) for 2-3 hours, then switched it off. A few hours later, powered-up again and just got a grey screen, followed by Apple logo and spinning gears, then machine switches off. Have followed all fixit suggestions from Apple support and various forums (inc this one), but nothing works.

The only suggestion that does not produce the greyscreen/apple/gears combo is holding down the Alt button before pressing the power button. This produces th same greay screen, but with two boxes/icons in centre of screen: HD icon and Disc icon (for install disc 1). Unfortunately, selecting either option just returns me to greyscreen/apple/gears and the machine switches off.

Can someone help, please?

NB During one of my unsuccessful atempts to power-up, I inserted Install Disc 1. Cannot now eject it.