Look at the unused wire stubs you've clipped. Perhaps one is touching another and shorting? Also check the removeable fuse to see if it has been blown. When you say it twitched, are you meaning that the cooling fan started to spin and then immediately stopped? If so, that sounds like a short. Perhaps you could take a soldering iron and a solder sucker and remove the actual remaining snippets of wire, so there's nothing there to accidentally short. Another possibility, are the posts you installed in the case touching the case itself and shorting?