As someone suggested, have you put the motherboard on the pegs/legs. Try taking all the additional components out leaving the CPU and memory attached see if it will boot. If not check the CPU is properly connected, it should still power on (at least the fan), even without the memory in, from what I remember.
Did you get a little speaker, if so plug that in and check what type of beep(s) if any you get.
If none of these help, take the motherboard out of the case put it on an anti static bag and see if it will boot.
You could check your jumper settings in the manual, but most motherboards don't have jumpers now, only to clear the CMOS which you can do by removing the battery.
If it will boot outside the case its shorting somewhere.
That should at least give you something to start
Anyway I need sleep, ill try have a look at this thread before work in a few hours, good luck...
Bann