I would take everything back to basics, so get rid of everything that is not essential, and then test to see if the issue still occurs. If the issue dosen't occur then start adding all the extras one by one testing in between untill you hit the component thats causing the issue. If teh issue still occurs then have a look in the windows event log to see if it gives you any hints.
IF it gives you some specific let us know, if not I would then do a fresh install of a different OS if possible, even better if it is a 32 bit OS. Then see if it happens then let us know how you get on. Also when you reinstalled the OS in what order did you install the drivers?