Just to help clarify the situation. The new motherboard is fitted with a PATA hard drive, a SATA DVD drive and a PCI express video card. The new motherboard also has the P4 connection , and the new PSU (500 watt) has a P4 plug .
The new PSU will not POST the new motherboard but will post an older motherboard that does not have a P4 connection. A second hand PSU (300 watt) that has a P4 plug will POST and boot Vista on the new motherboard.

I will be fitting the new PSU to a friends PC to see if it will POST/boot. If it doesn't POST then it must be the 12 volt rail at fault and can be sent back for refund. If it does POST then I don't know what to do.