Computer doesn't start after installing 550 TI

Hi guys,

So recently I bought a 550 ti from a friend, and I was excited to put it in then I realised I didn't have a 6 pin molex cable. So I went to an computer store and bought a 2x4 pin to 1x6 pin molex.

I connect everything up and insert the 1x6 pin molex and turn on the power supply.

The light at the front turns on, but then when I actually start the computer I get about 1/2 second of functionality but then the computer switches off.
After unplugging the gpu, the computer works fine, as I am using it now.

Can someone find out my problem?
Here are my computer specs:

H61M-USB3-B3 GIGABYTE motherboard.
i5-2400 Intel Processor.
1 Seagate 1TB harddrive.
1 DVD/CD combo drive.
1 CPU Fan
550W GHPOWER PSU (Shiftiest brand I know, I can't find anything about them)
8 GB ram.

I tried to power the graphics card with a seperate extremely old psu (280W)
No luck, the psu fan doesn't even start.

I am waiting for you answers :) 

Well the obvious thing is that the psu can't handle the draw. Another option is that the adapter isn't working. Another option is that you need to uninstall past graphics drivers first.