PC not booting, fault unknown

[BRiiTASH]

So heres the problem. My grandfather has a business and their cctv computer crapped up basically. Being the grandson who "knows computers" (basically ive built a few custom gaming pc's, cutsom watercooled them, stuff like that, I was basically the one to call on to fix it.

Anyway when I got my hands on it, I checked it and this is what I was greeted with.

They have some construction works going on outside and apparently that kicks up a lot of dust, but im betting some of this is due to the fact it prpbably hasnt been cleaned in the last decade or so.

So i cleaned it up (cpu block and fan, gpu, case) and reapplied some thermal paste, and it looks much better.

I havent bothered with cable management atm as its still unfinished until i can get it working.

 

Basically I turned it on and,was suprised to see it post, what with some of the parts being from around 2001, and the operating system being windows 2000 so nearly 2 decades old.

The problem was that after posting, the windows start up would hang at about 60%. I bought a windows 2000 SP 4 cd with key off eBay, changed boot priority to CD, and restarted. The pc overcame the 60% hang, the screen goes black, then comes back at like 98%, then hangs again. The little bar along the bottom continues to pulse left to right, but the keyboard becomes unresponsive.

I have tried everything I can, even booting in safe mode, safe mode with command prompt, debugging, directionary restore, last known good config but nothing seems to work. Even safe mode appears to hang. 

I have tried booting with the DVR card removed, reseated the gpu.

My suspision is that the gpu may have overheated as it appears there is some artifacting on the windows 2000 loading screen.

I am uncertain however if this would prevent the pc from booting, even from a bootable cd.

I have offered to build him a new pc for as cheap as i can get the parts, however it would be more convienient for me annd him if i could just figure out how to fix this one.

The pc wasnt connected to the internet, and was running pretty much 24/7. I suppose it could be component degredation due to age or mistreatment that some component has failed, but i cant figure out which. My money is on the gpu, but i cant be certain.

Does anyone here have any experience with a similair issue, or any suggestionf for me on how to fix it.