PSU Help new TX650 will not work

Okay so first off I am a newb when it comes to working or even dealing with computers so this may be a simple problem.
So I purchased a Gateway DX4860-UR15P Desktop, came to realize the integrated graphics card just wouldn't work so I set about trying to upgrade it. Since it only had a 300w psu I got a corsair tx650. I tried installing it and it wont work. When I press the power button all the fans start up for about 5 seconds and then everything shuts down. No sounds or anything. After I press the power button it just keeps trying to reboot about every 5 seconds but with nothing. This is the second tx650 because I thought the first one might be faulty. I tried the whole paper clip thing and the psu fan starts up and running but it doesn't stay on. ie shuts off after about 10 seconds. Any help as to what is wrong or how I can fix it? As I said second tx650.