Jakob Barrett :
After a mistake with Easytune6 earlier, I replaced the Motherboard with a Gigabyte GA-z77x-ud3h, same processor: i5 3570k and Thermaltake Smart750w psu. The MoBo light comes on and the fans spin slightly when the power comes on but everything shuts down seconds afterwards.
This could be an bitch one ....
I suspect you have BIOS lower than F8 and you mobo can`t power the CPU -
http://www.gigabyte.com/support-downloads/cpu-support-popup.aspx?pid=4153
Core i5-3570K is supported after F8
You can check you BIOS ver. if you add OTHER CPU that is supported and that`s the bitch
then you can upgrade BIOS to F18 or F19e ( latest one )
Then you add you Core i5-3570K back and there you go.
in adition :
Check all PSU connections - 4 pin CPU , GPU 6 - 8 pin
Disconnect all but Mobo + GPU + 1x RAM - faulty DVD or HDD maybe
If you can test with other PSU also