Picture posting is enabled for all :)
Most MBs have flashing lights on them when they're working. Take the side off and see what's happening when you switch on. Any chance it could be the hard drive?
i had a problem like you describe. It was caused by a USB mouse. With no mouse the computer was OK. With the mouse attached it didn't boot. the problem appeared out of nowhere ie the mouse initially worked fine with the PC. Have you tried unplugging all the peripheral devices?