Hi,
If your PC won't start, even with the Firewire header disconnected then I don't believe that is the cause of the problem and we should be looking elsewhere for the solution.
Could you post your full systems specs to help with troubleshooting?
Do you get any POST beeps?
Is it a new build or have you had the PC running for a while?
What were you doing when the problem first appeared?
I am sorry to keep asking questions but, for me or others on the forum to help, the more information you can post the easier it is.