Running 64 bit version of Win 7 Pro with current updates. Recently have encountered lockups with mouse freezing and pc inoperative - but keyboard remains operative. It seems to occur after pc has been running for substantial durations (2+ hours) and is sporatic. I can only restore operation if I do a power shutdown and restart. I realize there could be multiple causes, but wonder if someone could suggest the more probable reasons for this problem - where to begin and possible fixes.