Probably the PSU.
Our standard checklist and troubleshooting thread:http://www.tomshardware.com/forum/261145-31-read-posting-boot-problems
Just because the fans run does not mean the system does. All the running fans indicate is that the PSU is producing some small amount of 12 volt power and that you do not have a major short. A major short would crowbar the PSU and nothing would work.