Not sure if this has been mentioned in here but my PC wouldn't boot when I had a broken USB port where one or more of the 4 prongs inside of it were shorting against each other or the metal around the port. Prevented it from POSTing and no beeps. Straightening out the prongs and making sure they weren't touching anything fixed the problem. Might want to drop that in there.
I can see that being a common problem as it was caused by the black plastic piece on the inside breaking out. Lots of people aren't very careful with their USB ports (myself included) so might be good to mention.