Do you know whether the hard drive boots even when there is no display? Does the light come on and keep whirling away?
The reason I ask is it sounds to me as if some item in your PC is having trouble starting up. It could be the hard drive. A sign of a failing hard drive can be that it has problems starting from cold but once up and running it appears to be working fine.
Do you know that make of your hard drive because a lot of hard drive manufacturers have tools you can download. Seatools for Seagate, Maxblast for Maxtor drives etc.
Do you an old graphics card you could try in case its that?