Here's a good summary of the different water treatment techniques, with some info about Giardia:
http://www.hikelight.com/18ultralightbackpackingwatertreatment.htmlI myself used an eyedropper that I filled with bleach, and added a few drops to each 1L bottle that I filled. There were plenty of mountain streams that I would have been comfortable not treating, but it was just part of my routine.