What might have caused this?

I was playing Lord of the Rings Online this morning and the game froze, but after it unfroze there was all this weird jaggedy lines and pixelatiion and many other types of weird shapes and odd things appearing on screen. I thought it might just be LOTRO so I closed the game and opened up another, and as the opening credits began to appear on screen similar things were happening, the animation was all pixelated and there almost seemed to be a small bar of static on the middle of the screen. I then tried a system restore to just a few days ago and now the games play fine. I was wondering why this would have happened and what might have caused it, is there anyway to not have this happen again? Thanks.

here is an example of what happens sometimes when the game freezes.

http://i253.photobucket.com/albums/hh76/Shagy1186/ScreenShot00000.jpg

most of the time to fix it, i go into windowed mode and back to full screen and it goes back to normal.