It could be anything from a video driver to a back door trojan. In the case of the trojan , like sub seven, they would be doing a screen capture. More than likely it is the video driver. What os do you have ? ...and how often does this happen and what are you doing when it happens ? you said you just reinstalled the os ? did you do that or someone else, was it an oem or retail cd or was it a "burnt" copy from the net ?