Is this due to my lack of graphics card or have I messed up putting it together?
Could be.
It could also be that you put the connections between the powerbutton and the motherboard in wrong -- if those leads are in the wrong spot you might hit the wrong pin or complete the wrong citcuit, which can register as you holding down your power button continuously -- which on an ATX system will power it down after several seconds.
It's highly unlikely that the HD has anything to do with it, since those are not necesary for the system to POST. At worst, it would halt at a 'no boot device found' prompt or try to boot from the network or something.