Thanks for your replies. I did think about using another Windows XP CD but I haven't got one - still happily using WIN2K here (if it ain't broke don't fix it) + there's the problem with the BIOS that may have been tampered with - did a Google on it and found loads and loads of site/people complaining less that politely about it.
With the new mboard there will be a new CPU/RAM and a fresh copy of Windows XP - cust is happy to go down that route + salvage what we can from the old PC.
I got an actual picture of the back of the PC:
On closer inspection I agree that it looks like a mini-ATX - glad I asked!
Novatech carries a couple of examples so I'll direct the customer to them.
Cheers guys,
Charles