I am building a wind generator with a car alternator without a gearbox. I know that the alternator is designed to spin at 5-10k RPM and the blades runs at 60-120 RPM so the alternator generates less than 1 volt. My question is: Can I use a voltage multiplier such as a Cockcroft-Walton generator to get useable voltages at reasonable efficiencies?
You probably need to connect your field winding up to get more output but running at this dismal speed is going to be a poor generating solution. No, a cockcroft walton multiplier won't help much here because they need several volts p-p to make any headway due to the internal diode volt drops. Drop the idea of the CWM and stick with understanding how to set the field winding current. I believe a lot of automobiles use the field winding current to control output voltage hence the final alternator output is regulated.
If you are not using the field winding then you are relying on residual magnetism in the rotor for generating your output voltage so it will be low.
Alternators on cars need 2000 to 2500 rpm to begin charging, this is why the pulley diameter is about 1/3 of the crank pulley.
Also the ratio of the pulleys is chosen so the max rpm of the alternator is not exceeded at max engine rpm - the rotor can contact the stator and seize since the clearance is minimal...
You should look at gearing or making your own alternator.
Here is a link to some really good information about making wind turbines, and yes, this guy has been there and done it... http://scoraigwind.co.uk/a-wind-turbine-recipe-book/