I have a dead iMac 24" (Early 2008) that has a broken graphics card (and doesn't boot up to any system), I was wondering if I could convert this into an external display for my Windows PC, because it would be a shame to scrap it. If I can't then I guess I'll just take the hard drive out and scrap it.
I believe this should be possible. However, you cannot do this without changing the hardware of the computer. What you can keep though is the display and the housing. You need to take the rest of the computer out and take a look at the back or side of the display. Somewhere around there or on the display's flex cable you should be able to find a code which you can use to buy a compatible inverter board on the internet (just google or search on eBay, it should not be this hard). Now you need to plug your display flex cable into the inverter board. Then you will find a DVI/HDMI/VGA port on the inverter board, depending on which one you bought, which you need to lead to the outside (p.e. with an adapter cable). Now you can plug your computer into the port and use the iMac as a display. Some German guy did something similar with a modern 5k iMac with the only difference being that he bought the housing and the display specifically for it instead of recycling a Mac. Even though you probably do not speak German, this might give you a better idea of how it is done: https://youtu.be/hzTVe6aVgww And there is also an English video where someone tried this with a Windows laptop: https://youtu.be/CfirQC99xPc
Good luck! It is always better to try to reuse old and broken electronics instead of throwing them away!
Sorry, but that won’t work. As I understand how Target Display Mode works, the Mac has to be able to boot for it to work as a monitor.
Maybe you could find a used graphics card on ebay or even sell it there too. They are still popular and I might buy it but my wife might kill me! :-)
Wow, 2008? That's before Target Display Mode existed, before DisplayPort caught on, and before USB 3.0. It might be possible to open it up and find the cable that connects the GPU to the display, hope that it talks DVI, then wire that to the mini-DVI port on the back so you can feed in video that way. Replacing this with a "comparable" display today might be as little as $100 to over $600, depending on what you consider comparable. That explains the desire to make it work as a display.
Deciding on this being possible depends on how much you value your time, value the display in the computer, and believe in your skills in computer repair. It might be worth something in parts to someone, so you may be able to sell it to cover some of the replacement costs. If it's not booting though then it's likely not all that valuable to anyone.
If it were my computer I'd be reluctant to toss it too, and find a way to keep it running in some manner. How hard is it to open up? Have you looked around inside to see if there's an obvious input to the display? If the insides are undecipherable to you then repairs might not be worth your time.