DJBPace07 (thank you): This all came about because of problems I had http://www.bleepingcomputer.com/forums/t/312733/the-lights-are-on-but-nobody-is-home-psu-problem/
I ended up buying an exact replacement MB. Fitted it last night and still ended up with the flashing green light on the PSU and a flashing blue light on the front of the tower.
I did intend to keep the same CPU and RAM, looking on line for answers, I'd decided the problem did not lie in the PSU and I was replacing the motherboard... but after last nights dissapointment, now I'm not sure what the problem is at all!
from HP site, CPU - Socket: 939 Supports AMD Athlon 64 X2 (up to 4800+) and Sempron (up to 3200+) processors.
RAM - Four 184-pin DDR DIMM socketsSupports dual channel, eight memory banks DDR 333/400 DDR DIMMsSupports 2.5v DDR SDRAM DIMMsMaximum HP/Compaq approved memory is 4 GB* without ECC