Thanks, TextureDnB.
"I see alot of problems with RAM try booting with only one stick, should that fail try the other stick."
There is only one RAM stick (new). I'll try it in the other slot. If that makes no difference I'll have to raid another computer for a different stick.
"Do you have any external devices connected beyond monitor, keyboard, and mouse?"
On a couple of tries I had a router connected through the on-board LAN port, but nothing else.
"Is it actuallt hanging right after POST? Therefore no BIOS? This happens often with bad/incompatible hardware configurations and you have to boot into safe mode in order to identify the offending device/driver but if you are hruly hanging at the end of POST which would be BIOS detect drive or equivalent safe mode will be unlikely."
There does not appear to be a hardware problem as I installed and ran XP Home OK and, as you say, it hangs before drivers are normally loaded.
"While writing this I'm begining to wonder if this could be BIOS related considering the point of hang have yoiu checked for an updated BIOS? It's possible that you BIOS doesn't know what to do with the newer Vista Bootloader."
The BIOS is dated June this year, so I hadn't checked for a later one. May have to try that. I thought there might be a BIOS setting that needs changing, but couldn't find anything that seemed relevant. Seems to be missing the boot instruction - whatever that is in Win7
I'll try the RAM things and see what happens.