Microcode failure - Update Bios - contact HP

Hello,
I'm rebuilding a HP Pavilion Desktop a1200a. All I had to do was add a PSU, Intel 2.66GHz processor and hard disk. These items all worked fine in another PC. One compatibility problem I had was the Psu is 20 pin and M/B 24 pin. After some research I was told it will work and to insert the PSU plug into the M/B on the right hand side of the plug leaving 4 plugs free on the left. I will use on board graphics - there is no PCIe graphics slot.

I booted up to install XP Pro and an error appeared in the post..."Microcode Failure...update bios...call HP" I contacted HP and they didn't want to know except to say I couldn't update bios unless I had a functional O/S. I thought I could do it with USB flash drive or CD..."No" was the answer.

I F2'd to continue out of boot and access CD. Install worked fine right through the process but when reboot time came around I got the BSOD. I've gone through the whole process about 7 times with the same result. Could the fact that I'm not connecting to the M/B with 24 pin power lead be the trouble? It's happened with 3 different hard drives (2 x IDE and 1 x SATA)...every thing happens exactly the same from Microcode failure to BSOD. I could buy a 24 pin PSU but if this isn't the problem it's a waste of money and I don't have access to one to try from another source.

These are the files in the latest windows XP based HP bios update .exe file
amifldrv.sys
AFUWIN.exe
315.ROM
installer.exe

I had a brainwave! Could I rebuild my Windows XP installation CD with NLite and slipstream the latest HP bios update .exe file into it?

I once had a bios issue with a notebook where it wouldn't recognise SATA drives. I NLited the files into XP disk and it worked loke a dream.

Any thoughts chaps?