Okay I need major help? Sims 3 and computer problems?

Okay so, I LOVE to play the Sims 3. And I've been majorly mad for the longest time, because I bought the Sims 3 the day it came out and when it came I was so excited and I put it on my Dell Windows Vista computer, and for a while it worked GREAT! But then in the middle of the game it would all of a sudden freeze. I thought it was nothing for a while and just started playing again. But then it started to like.. I don't know how to explain it? Like it would freeze and then all of a sudden the screen would go black. Like just blank. Then the Sims would pop back up, but MUCH less bright. It was really dim and all that jazz. Then, it would do it repeatedly like 5 minutes after the last one, and then it just shuts my computer off? Like it makes it restart and everything? So I want to know whats wrong and how to fix it? Because I haven't been able to play it in forever without it messing my computer up. Oh also when the computer comes back on the icons or whatever will be like HUGE! Also this never happens with any of my other games, like Chocolatier 2 or anything like that, I can play for hours on those and my computer is fine. Also it doesn't malfunction at other times. So is it a computer problem or a Sims 3 problem? Thanks alot :) Oh! And if it makes a difference, my computer is a desktop.