I would disconnect anything that was non-critical during a bootup. This includes floppy's, CD-Roms, ethernet cards, etc... Basically just leave RAM and a video card intact.
Then I would see if there is a setting/jumper to return the bios to its defaults. Usually you move a jumper over a pin, wait a couple of seconds, then replace the jumper to its original location.
Then I would power the PC on and see if it posts.