oh man, that could be SO many things :/
I have seen error code 43 for:
Bad riser
Riser not connected right
Too many display adapters in windows
Too many IRQ resources taken up (usually a USB device)
One time i even had error 43 as i had forgot to plug in one of the 6 pins on the card!
Check everything, switch out the riser, you likely just have a simple error somewhere