Issue with booting up PC

Hello,

So i built my own PC a couple of years ago now and since i built it some times it just will not boot, other times it's fine. I just bought some addition RAM for it and thought i'd give this problem another go since when i put it in i had to try turning the PC on about 10 times.

Basically what happens is i press the power button and the lights come on, fans go round etc but thats it. I can't hear the hard disk trying to access anything and nothing appears on the monitor.

I know it's not the network card and it's not the graphics card because both have been replaced since i built it, i don't think it's the RAM because as i said earlier i have just bought some more and it still failed to boot properly several times. To me it sounds like either a PSU or motherboard issue rather then a problem with the HDD because it is intermittent.

Any thoughts someone could give with regards to my issue would be most helpful, i just want it to boot up properly :/

FGN