Why is my iMac G3 rejecting the start up disc?

I have recently upgraded my imac 3G 400MHZ slot loading imac (blueberry) by giving it a new hard drive, and memory - both compatable with the model -and replaced the buzzing old speakers too. I tried to boot up from the CDs it came with in the usual way - pressing the C key, but it just keeps rejecting the cds.

I have tried booting it up using my mac laptop in target mode - the imac sees the lap-top hard drive but cannot access it.

I was really carefull to make sure all internals were firtted properly and wires all connected.

feeling really fed up. Can anyone help me please

thanks

Stephen