As long as the humidifier is in the same room as the PC it is bound to increase the moisture content in the air and this will probably result in condensate deposition on your casing and eventually lead to corrosion which is more harmful than the static dust on the casing which can be removed easily using a filter. If you want to still get rid of the dust and save yourself the hassle of vacuuming then a room air purifier is a better option.
Use a mid sized room air purifier. I use the Clair TD1866 model (http://go-clair.com/). I assume your room is well ventilated and there is decent air circulation . In that case a room air purifier will capture all that fine dust much before it settles on the PC and other electronic equipment in your room and you also have the additional benefit of raised indoor air quality minus dust,germs and what not ..