2 thoughts - first do you have any usb leads connected - I spent several hours with a non booting pc and by chance removed a usb lead and it all worked ok. Second - you could have a faulty power supply. Even though fans whizz and the hard drive buzzes some parts may not be receiving power. Check internally that all cables are connected - remove lead and then replace rather than just pressing on them. Finally a faulty component ie floppy; cd or memory module etc., could be blocking things. If all else fails remove the leads to all components and see if the pc will boot at all. You do not mention if you get any beeps at start up these often pinpoint the problem.