Can be caused because of overheating of the graphic card.
Educated guess...the graphic card is not the best...you are running windows vista and a lot of background processes...I think your comp is really on the edge.
What graphic card do you have? We could recommend a driver update after that and I don't think that installing HEK has anything to do with it.
![Confused :?](./images/smilies/icon_confused.gif)