Make sure all CD's, floppy diskettes, USB drives, etc. are out of the computer. It's possible it may be trying to boot from one of them.
If that doesn't help, have you recently added any new hardware or moved the computer?
If not, are you able to enter the CMOS setup (pressing F1/F2/Del) as computer boots up?
It's likely if you're down to this point and same issue that there may be a hardware failure with the computer.
