Western blot help - why am I probing for actin the 'housekeeping gene'?

First of all, a housekeeping gene is pretty much a generic, omnipresent gene, that's found everywhere. The reason you probe for the actin is to correct for any differences in loading of the gel when you put your protein of interest on there. If one time you put a little more or a little less on the gel, then the actin will also be a little more or a little less also, so you can make corrections. It stays constant because it is just a common gene product that is everywhere, and really doesn't change unless you make it change.