from what i have read it is difficult to lose weight and bulk up at the same time.

however, since protein is mainly used to repair muscle damage i dont think it will have a negative impact on ur weight lose particularly as most protein shakes are low on carbs and fat. and if you have it after ur workout and with water it will be abosorbed by the body quicker and therfore less likely to be stored as fat. Also since you have it after your workout means that the body will have to use fat reserves for energy, thus burn off the excess fat.

ive not been training long myself so if i am wrong im sure someone with more knowledge will be along shortly to correct me.