what voltage did you use when you over clocked the old p4?
on the older p4's if you put too much juice through the cpu the cpu eventually started to run slower than it should. In effect the cpu was dieing, Is it possible this could be the cause of what seems to be its demise??
From reading what you have done, it should clear the cmos.
As for flashing the bios this should be done from a boot floppy on such an old machine, I doubt there was a win flash bios available for that machine.
Could be worth checking to see if you can get a newer bios download it and boot up from it using the pulled plug from psu method you have been using to get it to boot so far