Could be any number of things?Dodgy IDE cable,flakey memory module,hard drive problems,corrupted software,I certainly do'nt envy your current predicament.Good luck.
Try fresh install of os,complete drive reformat that means ALL of tour hard drives and minimal software installation and I mean minimal being only necessary drivers+ programs that you MUST have.Run like this for a few days and take note.If there are no probs then start installing whatever else you need,but keep taking note to see if any new installation causes any probs.
If there are no problems after fresh os install,chances are its software,if prob persists after fresh os install then chances are its hardware,which means the old process of elimination beckons you on it's merry journey.Happy travels.