just having a read through this, did you get sorted ferg?
had a problem myself during a reinstall recently, seemingly vista doesnt like if you have more than 2 gs ram installed while doing updates from scratch, bloody nightmare
i was gonna suggest a reflash of the bios, itll reset your oc settings alright but wont harm whatever you have installed, something maybe to try if the memory tests prove ok and if the ram is oc'd too.