Cannot Startup after attempting to install unsuported windows, Please Help

I have an iMac 20 CORE 2 DUO 2.0GHZ 1GB 250GB DVDR HD2400XT 128MB
It was running on leopard

I recently tried to install an unsupported version of Windows installer via Boot Camp. Now when I switch the machine on there is just a blank white screen.

The mac won’t boot from the hard drive, or from the installation disk. I have tried booting with all the recommended boot key combinations, zapped the PRAM, and VRAM. I have
tried running the firmware restoration disc. Nothing gets a response. The mac won’t open firmware on start up.

The only things that get anything other than a blank white screen are

If I press X on start up I get a black screen with a white dash on it.
(I cannot type anything)

And most strange I have a backup copy of the operating system on a DVD
which was not burnt as an image copy DVD but burnt from the opened
programme. If I put this in and press option on start up a disc
appears with the instruction to EFI boot. When I try to EFI boot with
the disc in the apple appears and the cog showing its trying to boot
up but it doesn’t boot. This only happens with this copy not with the
original disk.

I was considering opening it up and removing the battery, hoping to
restore everything to factory settings, but this is not straight
forward.

I have been on the apple web page which does cover this issue, but recommends going to a service provider. I have been quoted £150 to fix it.

Any advice on how I can sort this myself would be much appreciated