Can I retrofit a wind-power system for my home?
Here are a couple companies that make rooftop wind turbines 


Swift Wind Turbines.
Wind Energy 7.


Not sure if these things would require special zoning, permits, etc. or if they would even be allowed in all locations. A call to your local government would be the easiest way to find out if something like this would be permitted in your area. You may also have to check with the homeowners association; if that applies for you, as there may be noise or other issues with installing a wind turbine on the roof (Those homeowners folks can be awful finicky).

EDIT

Here is a site with loads of "small" wind turbines allsmallwindturbines.com, you may be able to find an appropriate one from this list.

Finding a very small (&lt; 6 ft./ 2 m) turbine may be difficult, as the blades have to be of adequate size to be able to harness enough energy to generate power.