I have been given a dell dimension e520, can't get it to run. When I turn it on I get one beep but I can't get a display. I have reseated the RAM, removal results in constant beep. I've been booting up with just one stick of RAM, no drives or cards. Dell's diagnostic lights on the front of the PC indicate a problem. Lights 2, 3, & 4 stay on solid green, no flashing. According to dell this means "possible usb failure". Any ideas?