Registry stop error - how to diagnose

I'm getting a registry stop error (blue screen) and can't figure out what is causing it. Here's what I've done so far.

I have two pcs which are identical so I've switched out the harddrives and it boots fine but after a few minutes the blue screen appears with the stop error. I disconnected the cd drive as well incase that was the problem. I thought maybe the RAM was corrupt so I switched those out as well and got the same results. I think it might be the integrated graphics but I don't want to purchase a graphics card if that's not the problem. Any idea how to see exactly what device is causing the problem or is it just a trial and error?