will not boot

Hi, i've got a 24 inch imac 2007, i found it with a grey screen with the apple logo and spinning wheel beneath and it won't boot, so after serveral attempts i decided to re install with 10.8.

it was fine for 48 hours and then done the same thing, so i replaced the hard drive and reinstalled again, now it done the same thing.

i've ran out of ideas.......any one help?