I suspect a short circuit between the mobo and the case. Breadboarding would help determine if there really is a short. Follow these steps:
1. Disconnect all wires (except for the heatsink fan).
2. Uninstall all connected devices (except for the CPU/HSF), including the PSU.
3. Remove the mobo from the case, and then place it on a non-conductive surface (like the mobo box).
So, now you should have a bare mobo (with exception of the CPU/HSF) on a non-conductive surface. To breadboard, do the following:
1. Install just one stick of RAM (consult the mobo manual for single stick placement).
2. Connect the PSU
3. Connect the monitor to the integrated video
4. Using a flathead screwdriver (or similar metal tool), jump the PWR_SW pins on the mobo.
If successful, you should be able to POST, but it will fail because there is no HDD connected. If this happens, shutdown and connect the HDD. Retry.
If successful, you should POST, but it will result in an error message indicating no OS or boot device could be found. Install the OS. Now, connect the case power button wires to the PWR_SW pins, and then retry.
If your first attempt resulted with the symptoms persisting, move the RAM to a different slot. Repeat until you've either booted or tried all of the RAM sticks in all of the slots.