Double check nothing is obstructing the fan blade from turning ie. wires cable ties etc. Have you a molex connector on your PSU cables ? Could you connect it even for testing ?
Or disconnect the single wire connected to the motherboard and swap it with the one 'hanging down' ... see if the big boy fan starts and rear fan stops
sorted, it did do this wierd black screen when i started it up after it though, i got onto the desktop and the screen just went black, it didnt lose signal, or anything, it just went black, restarted now though and all going well, thanks for your helps 