Checksum error is usually a corrupt cmos or memory problem Dan.
I came across this earlier so I am surprised this guys solution that I suggested at the start didnt work for you
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I just built a system and when I turned it on it counts the memory and identifies the IDE devices, but I get a message at the bottom of the screen that says Checksum error. The system won't let me go into setup and I don't understand why I am getting this error, I've never gotten it before.
It was the first time I had ever seen it, and when the error came up the delete key that you suggested to press didn't work. My keyboard would not function after the error came up, so I couldn't even enter the BIOS. I just cleared the CMOS and checked the battery and when I rebooted it was fine.
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Unless its like Dack suggested and the plug is fouling the case then I am at a loss of what to try next
