So it turns out that I am a moron. I know this is a shock to you all.
The only thing I had added into the configuration since it was last working was the network card.
I had apparently completely forgotten that fact... Naturally removing it resolved the issues immediately...
I finally arrived at this after first eliminating absolutely everything else... I tried installing with half the RAM slots populated, I tried the other RAM slots, I tried the other RAM sticks, I tried one RAM stick, I tried another GPU, I stole the SSD out of my laptop and formatted it (good job me!) to try that, I tried another optical drive, I burned another copy of Vista to another disk, I even changed some BIOS settings, and ordered another SSD, (order since cancelled...) before finally... It dawned on me...
I must have half installed Windows to and re-formatted that poor Gigabyte SSD about a dozen times...
So... The onboard NIC still doesn't work. It is detected, and the drivers automatically install, but it doesn't detect when a cable is connected.
I found the Broadcom chip on the board and had a look at the condition of the components around it, and there was a tiny surface mount resistor that looked a little crooked. I gave it a gentle prod, and it fell off...
Circled here in red:
So, thats quite likely to be the issue I'd imagine. I have a Microsoft Surface USB to ethernet adapter somewhere, I'm going to see if I can use that... If I can, problem solved as far as I'm concerned!
Before I resign myself to that solution though, I am going to see if I can install the drivers without the network card installed, and then see if it will boot into windows and work. Fingers crossed... I am not holding out hope!
e; That didn't work, though the BSOD didn't occur when the card was installed in the second PCI-E x16 slot. Regardless, I can't make the drivers work anyway, so USB thingy it is... It better work or I might lose it.