PC turns on from PSU switch (not power), only stays on for 2 seconds

PC turns on from PSU switch (not power), only stays on for 2 seconds

Hello, I hope someone here can help me with this.

I recently just got parts for my girlfriend's mom's computer. Ive built plenty of high end machines but this one she only had a budget of $400 so I put together the following build:

MB ASROCK|H61M-DGS HD61 LGA1155 R

CASE COMPUCASE|6K28BB8F RT

VGA MSI|R4350-MD1GD3H/LP HD4350 1G

PSU ROSEWILL|RV350 350W RTL

CPU INTEL|CORE I3 2120 3.3G 3M R

MEM 4G|CORSAIR CMX4GX3M1A1600C9 R

HD 500G|WD SATAIII WD5000AAKX %

DVD BRN ASUS | DRW-24B1ST/BLK/B/AS%

I've double checked that everything was plugged in. After that I turned on the PSU switch in the back to initiate the power, but when I did that, the power of the computer went on, for about 2 seconds, then turned off. Any time I tried to press the power button in the front of the PC, nothing happened. I tried changing the position of the power button cable to all the different slots on the mobo just to ensure that mine was on the right two pins. But, no avail. The only time the computer starts now is when I shut the PSU switch off for a few moments, and return it back on, but it only runs for 2 seconds and automatically shuts off!

I've never experienced anything like this. My only guess is maybe that the power button on the case is shorted? but if that's the issue how could I figure it out? Its not like I have another spare case sitting around.

Please help, the last thing I want is my girlfriend's mom on my case