Black screen?

[Dusk Magpie]

Every now and again when I'm playing it will freeze then the screen will go black. Does anyone know what could cause this?
I can press the windows button to get the start menu up, that works, but then when I try to go back on the game, sometimes it will just go black again.
I tried getting a new graphics card but that made the whole game(and computer) slower. >.<

 

[bighairy dog]

That happens to me all the time almost every other time i turn the game on. I dont know what causes it but pressing control alt delete only freezes the comptuer when it happens.