It sounds like a possible glitch with the boot information or possible wear and tear on the hard drive itself taking a toll. When setting a drive here with no OS as the default boot device all you see is a small flashing cursor in the upper left hand corner of the screen. That was before Vista went on the second sata drive formerly used for storage now that an external drive serves that purpose.
No post screen at all proceeds a drive problem however like a need to reseat the video card if that isn't going on you. These are the two more common hardwares if it isn't a minor connection problem or the card is now lifted up in the slot a little. Have you used a can of air cleaner lately? Sometimes dust and dirt buildup in and around the slots on the board will tend to cause problems.