Oh, I forgot to update the thread. Yeah, it was the motherboard. Swapping that utterly jiggered Windows but it so happened to be the Windows 10 release day so I had an excuse to do a clean install (of 8 so I could upgrade to 10) anyway.
The shouting and screaming was particularly effective.
Thought as much, it's always worked for me
Sounds like Fozza (well, maybe not quite..)
Are you sure you didn't over-volt the AGP slot?