If you've swapped out the HDD then it must be either mobo or cpu problem. If it fires up so that you can hear the fans going then I doubt it's a psu issue. If you've made any recent hardware changes, then undo them.
Also, it may pay you to strip the mobo of it's add in cards one at a time - for example, faulty ram will prevent a boot up, but I'm thinking graphics/sound/tv/modem cards etc. Removing them one by one and trying to boot the system may indicate a fault in the card, although in all honesty that is unlikely to be your trouble. Still, doesn't hurt to try these things.