Start with the basics. Did these machines ever run or are they new? If they never ran you probably have some kind of conflict, like they are both trying to boot off of two drives at once. If the have floppy drives I'd start by unplugging all the drives (hard disk, CD, whatever), and see if you can boot off a floppy.
If it will boot then you can be pretty sure it is some kind of conflict.
Check your jumpers on the drives, the primary HD should be set to Master.
Also check the boot sequence in the BIOS. The system may be trying to boot off a network and it's unlikely you want to do that. Assuming you are loading Windows off of CD you probably want the order to be CD1 and then HDD1.
Also if you have more than one hard drive, and one is a regular drive and one is a SATA drive that can also confuse things. This computer gave me fits when I built it, and it was doing exactly what you describe. I wanted to boot off an IDE drive but with the IDE and the SATA drives connected it was trying to boot off of both of them at the same time, and that just didn't work.
Now if these are older machines you might have another problem, such as bad capacitors, that is a HUGE problem on the later PIII?s and earlier PIV?s.
You might give us some more detail, what processors you have, what motherboards, type of hard drives, how many drives and what kinds?
Good luck!
Jeff DeWitt