Computer doesn't start

[Ben137]

I put together a new computer yesterday (Gigabyte motherboard, Phenom II 720, 4GB RAM with an Antec 300 using my old hard drive and graphics), but it doesn't 'post', all the fans will start up, but nothing else (Monitor, mouse and keyboard).

I tried changing and removing the RAM - so doubt its that, also tried disconnecting the HDD but can't get it to work, fans start but never any beeps, could a faulty PSU or Graphics card cause this or is the motherboard faulty?

Thanks

 

[Ben137]

Yep, they are both in