I did my first slim the other day, and it was fine out of the case, but stopped soon afterwards once inside. The middle long screw that holds the metal case to the lid was done up too tight, which pulled the dent in the case up, putting pressure against the hole on te motherboard. The same hole which had 2 wires running through. This caused the wires to get pinched and short against the case.
Just check that your not damaging the wires with the screw that goes thru the hole that the wires feed thru