assuming you've got everything connected properly (and that's not something we can readily tell you)...the only way you'll be able to trouble-shoot this is to use another socket A system and start swapping parts one by one.
i'd start w/the power supply since that involves the least amount of work and you might be that '1-in-a-500' that has a bad PS.
good luck.
ps. try removing the mobo from the case and place on a piece of cardboard to see if you don't have a short somewhere.