Sorting Out the Freezing, Restarting and Blue Screen Windows XP PC

My PC''s been doing this for the past month or so. Randomly freezing or throwing up blue screens when I'm just word processing or browsing the internet, restarting when I'm watching videos or playing games. Thought it might be a problem with a recent Windows update or it was just the system needing a re-install. Didn't work. So, next...