My computer won't turn off. It just re-boots when I try to turn it off

I just built a new computer. These are the parts:

Intel i5 CPU
Gigabyte P55 motherboard GA-P55-UD3R
4gb DDR3 RAM 1033mhz
Nvidia 4650 1GB DDR2 RAM
600 watt power supply
Cooler Master Cosmos 1000 full-size tower
3 hard drives totalling 2GB of storage
Samsung DVD burner
Vista Home Deluxe

I put the CPU and mobo into the case which use to have several other CPU/mobo combos - Phenom 9600 and ECS mobo; Phenom X4 805 and Asus mobo; Core 2 duo E8200 and Gigabyte mobo.

The problems that I have been having are:

1. That I cannot play high definition videos anymore. I was able up to this newest mobo i5 combo.
2. The computer will not turn off. When I shut it down, it turns off and re-starts.

I was wondering if it was the power supply since the power supply has a 2x2 molex motherboard power connector while the motherboard has the 2x4 connection. Shouldn't the power supply be powerful enough?

Thanks.

Sorry, I should have proofread my post for the glaring errors. Thank you xaira fo pointing them out.

I actually have 2tb of hard drive storage and the video card is from ati. Silly me.

Anyway, when I am trying to play hi-def videos with Pixela software, which came with my Canon HD camcorder, I had no problems before. Now when the video starts up, the computer locks up and I have to hit the reset button to restart it.

Also, when I exit the computer for Shut Down, it restarts.

Thanks and any suggestions would be appreciated. I didn't have any problems when I had the other cpu's. I switched because the Intel i5 is so much faster than the others and I really like the quad.

Unplug your reset button from the motherboard and see how that goes.