drive c: missing

I have recently recieved two pc’s to repair one of which is worth repairing the problem is that one is a gateway the other is a compaq the gateway is the “good one” the person that brought me these had a problem that the hard drive wasnt working so he formatted it and then couldnt get it to work again so he swapped the drives from both pc’s and really got things screwy now I am unable to get it to bootup even with a startup disk and am no longer able to get to bios to set the paramaters for thedrive that belongs to it he had also let his child pull the cmos battery out and in the process broke the clip that holds it in any ideas? maybe it is time for a new pc?