Windows slows and crashes very often

Hi all, I've been using my basic; custom built PC for a while now, about a 3 months ago I installed Windows Ultimate 64bit on my second hard drive; while keeping Windows XP on my first one, although within the past couple of weeks it has been slowing down and stopping for several minutes at a time, this happens at least 2-3 time in an hour. Sometimes the computer crashes and has to be restarted,...