First question is, why on earth would you want to output your desktop video to a smaller laptop screen.
Assuming you have your reasons for doing so, it's possible if your laptop has a VIDEO-IN port where you can connect an external source, and your desktop video card has a VIDEO-OUT (usually S-video) port which can output the desktop to an external device. Note: the video quality will be poor.
You can, ofcourse, easily use your desktop monitor in a cloned or extended dual monitor setup with your laptop just by plugging it into the VGA-out port of yout laptop.
Another option to actually control the laptop from the desktop (not just video) is to use the Remote Desktop Manager in Windows and login to the laptop from the desktop. Obviously both have to be networked for this to happen.