Home built Pc not switching on.

Hi, on friday my pc switched itself off and wouldnt come back on so I came to these forums for help . I was told my power supply had gone. Today I received my new power supply and it still wont turn on. When I have all the power cables plugged into the gpu the pc wont switch on but if I take one or both the power cables out the gpu the pc lights up and the gpu fans spin at 100%. My gpu was only bought in january and it cost £250 surely it cant be broken? It hasn't been over clocked in fact I had the clocks turned down as I only play football manager.

My pc specs are:

Msi Twin Frozr III 7950 3gb
Gigabyte 990xa-ud3 motherboard
Kingston Hyper X DDR3 RAM
Seagate hard drive
60GB OCZ SSD
AMD FX6100 cpu

Any help would be appreciated.