Disconnect everything from the mobo, cd-rom hdd floppy and try to powe it on, if you get no post then take out the memory and try to power it on. You could have a toasted peripheral or memory causing a short.
Literally, everything from the mobo. Take out any cards as well (except video) and then if no post take out video too. Basically keep removing parts until you get down to just a mobo, cpu and psu. if still no boot it is one of those 3 and if you have a new power supply then its proc or mobo.
Also depends on ther motherboard manufacturer aso to what the on board led signifies as far as power.
Note: if you have a floppy that is the first thing I would disconnect to start the elimination process.