Be sure to note what Tom said.

I just rebuilt my powersteering pump becuase it was funking up my drivers side fender like it was going out of style. It was so bad to the point that it was soiling my otherwise CLEAN! engine compartment.

Anyway, rebuilt it (couple o-rings and a front seal) and its all good and working now. If you notice the oil is dark and black like engine oil...then look for an engine leak.

If its light colored (or whatever color is in your PS pump) check that out...the back of the pully is where it slings)

If it is colorless (ie water) then suspect power steering pump and the like...but I doubt this since you are talking oil stains here...not water.

Pictures perhaps?