Those PC's likely do not have any true hardware acceleration/Overlay modes support on their videocards. They are likely generic, integrated video on the motherboard.
Hardware Acceleration, checked or unchecked, can just simply mean MMX/SSE support on the CPU. If the videocard/gpu in question offers no extended Overlay modes, hardware deinterlacing or other technologies specific to streaming video, the PC simply uses the GUI/2D mode + cpu enhancements (if any).
Also, the video drivers can be "kludged" to trap for printscn/video capture and sense if Overlay is the front-most display, to capture THAT to the windows clipboard/buffer. It's a nifty feature of some videocard drivers to "seamlessly" capture the right buffer space, but very few hardware companies incorporate that into their drivers.