Solved - Updated the motherboard bios to the most recent I could find and it now boots just fine with the RX 570 installed.

I just bought this graphics card to upgrade a PC so my daughter can play a few newer games - https://www.overclockers.co.uk/asrock-radeon-rx-570-gaming-dual-4096mb-gddr5-pci-express-graphics-card-gx-005-ak.html

The problem I have is that when I install it the PC wont POST, it give 5 beeps indicating a CPU error. The PC runs just fine on it's onboard graphics and I have tried the only other spare card I have in there (an old 8800 GS low profile card with passive cooling) and it boots just fine and displays an image, so I am assuming that the PCIE x16 port is OK.

I RMA'd the RX 570 thinking that I must have a dud card, but it tests out just fine with the supplier.

I am thinking it might be power related, the 8800 I tested it with is a low power card and doesn't need an additional connector from the PSU, unlike the 570. The power supply is a brand new EVGA branded 600W supply, so should be adequate, but I am thinking it might be faulty and it's output is dropping when the graphics card is connected.

I am just waiting for the card to arrive back, but in the mean time has anyone experienced anything like this or have any other idea what the cause might be?