unknown

Hi peoples. I've recently installed another 1gb of RAM in one of my pc's running on XP pro and it randomly reboots itself now. I originally had 1gb (two corsair twinned 512's)installed and it ran fine. I built another PC at the same time when i made this one, a twin of it actually, and decided to used the ram out of that one to add in this one. So ive added another twin pair of corsair 512's (exactly same model as originals) and now it randomly reboots. From what ive read about other peoples problems, theirs might be something to do with the timings, and that they might be different? but might are the same model of ram! so i cant understand what might be wrong. Any ideas please peoples? Thanks..James