Watermark-detection in the graphics card of a personal computer, for the purpose of copy-protection, has recently started to draw a lot of attention in standardization. Detection in the graphics card has problems completely different from the formerly considered detection in the DVD-drive, having to do with high data-rates, large scale-ranges and presence of multiple video-streams in the display area This invention proposes examining (36) the video signal being generated by the computer system to locate image areas in which the video signal changes from frame to frame, and defining (37) a bounding box around said image areas and considering an area of interest this found as being a window in which an application is running. After approriate scale conversion (32), conventional watermark detection (31) is subsequently applied to said window. This prevents hackers from rendering illegal contents in one window while playing back a compliant disk in the DVD drive (10).