restart problem because of graphics settings.

2 - 3 weeks ago, I got an ATi Radeon HD 3650 graphics/video card.I love the graphics output on it, but when I set the Anti-Aliasing level to 2X samples per pixel when I'm running a game, the computer just up and restarts. It doesn't do that when I set to 4X or 8X samples per pixel, but for some reason it doesn't accept 2X samples per pixel. It ust resarts. No warning. Just, nothing.Can someone help me?Thanks in advance.