pc startup problems

well im not sure what this would go under since i dont know whats causing the problem. but here it goes.

sometimes when i start up my pc (i have 2 pcs this is the one i dont use often till recently) it gets stuck at the gateway screen( the screen that normaly lasts less thena second, before the windows screen) and all i have to do is click the power button and it shuts off immediately and just turn it back on, this isnt good for it but its all i can do. i normaly have to shut it off and turn it back on 1 or maybe 2 times and it will go throught the loading process and eventually turn on.

does anyone know whats wrong?

this could be caused by dust buildup or by a failing psu or mobo. i'd clean out the inside of the case with a can of compressed air or something, unless your warranty is still applicable.