Hi there is no need to boil the milk the kefir grains/powders have much stronger strains than used in yogurt and out grow contaminants. You can use raw or pasturised milk, obviously the main difference is lack of naturals substances found in raw milk. However when making yogurt you do need to pasturise the milk by bringing it first to 180f then cool to 100 and add starter. Back to kefir though, you can culture it for 24hrs, I do but the lactose may not be as used up, tolerance builds over time though. So 48hrs may be better. A couple of URLs are below

www.chariot.net.au/~dna/Makekefir.html
http://www.healingcrow.com/ferfun/ferfun.html

Phil