seems like you have covered most things already,but here goes
Try booting with a windows98 start-up disk in the floppy drive. (you will have to set bios to boot A: first (if you get as far as the "press del to enter setup" or whatever yours is). if you dont get this far then try resetting the CMOS either by powering down and placing the jumper over the clear cmos pins for a few seconds then replace in the normal position and try again, or by removing the cmos battery, leave out for a few seconds then replace.
only other thing is to remove the IDE cables from the Motherboard and try to boot again, it could be a dead hard/floppy/cdrom drive that is stopping the boot.