look at how the motherboard attaches to the tray very closely
take note of how much room is between the motherboard and the tray
remove the motherboard and see if anything from the bottom of the motherboard could have been making contact with the tray
Take the memory out and put it back in
Take out the hard drive/cdrom cables, plug them back in
in fact check all the cables
Did the new case come with a power supply? If it did try the old one from the old case.