Blue Screen

I have a Dell XPS 410 desktop pc, and for the past 6 months or so, it has started showing the blue screen error. I replaced the hard drive and upgraded the graphics card as well, and I reinstalled Windows Vista on the brand new hard drive, and it didn't blue screen for a period of time. But now it is starting it back up again, and often it does this three of four times right away. Then it will say that the computer has recovered from an unexpected shutdown, and it will request me to search online for a solution. When I search, no solutions come up. A few people have told me that the blue screen error that I am having is a software error, not a hardware one. When it does blue screen, it automatically restarts. When I Googled the issue, it does show that it is a software error, but I don't understand why it keeps doing it, and yet does not find a solution when I search for one.