Your VGA card (driver) doen't support overlay in current resolution and colour depth. Try to set desktop color depth to 16 bit. If it doesn't help, try to change the Video output method in program settings, Video tab (Overlay/Primary surface), also try to change Video stream size. Update your VGA card driver.
The other reason of black screen is that some motherboards set wrong PCI Bus Latency Timer value in BIOS. In tab "Hardware" in Behold TV program settings you may change this value in Device configuration and so force to work PCI SAA713x videodecoder. The recommended PCI Latency value is 16-64.
Also in server versions of Windows OS partially or fully turned off video acceleration by default in order to increase reliability. This also results in "black screen" or color distortion while working in different video output methods. Open display properties, swith to Settings, press "Troubleshooting" and set hardware acceleration to maximum. Run dxdiag.exe utility ("Start" –> Run… –> dxdiag.exe) and check that the hardware acceleration of DirectDraw and Direct3D is fully turned on in "Display" settings of the utility.