I have no idea if the cables are interchangeable, but try booting Safe Mode (to "Safe Boot")
1. Be sure the computer is shut down.
2. Press the power button.
3. Immediately after you hear the startup tone, press and hold the Shift key.
Tip: The Shift key should be held as soon as possible after the startup tone but not before.
4. Release the Shift key when you see the gray Apple and progress indicator (looks like a spinning gear).
During the startup in Mac OS X 10.2 through 10.3.9, you will see "Safe Boot" on the Mac OS X startup screen.
During the startup in Mac OS X 10.4 or later, you will see "Safe Boot" on the login window, which appears even if you normally log in automatically.
To leave Safe Mode in any version of Mac OS X, restart the computer normally, without holding any keys during startup.
If the computer works normally and doesn't shut down after you have been running in safe mode for an hour or so, then you have some sort of software problem in your main account.
Your fans will be loud, and you will only be able to connect to the internet using an ethernet cable.
You can also create a new account and boot from that, again, if everything is normal, then there's some kind of corruption in your main account.
Message was edited by: MGW