agressive to others

I have a 5 year old cat, Callie, who is VERY agressive towards other people. She is lovable when with me but the minute someone else is here she loses it. She hisses, screeches, spits, and bites. Since I have had her it has just been her and I living here. When my mom visits Callie will attack or won't let her up/down stairs. She is awful at the vet..so bad in fact that he gave me tranqilizers to give to her before we go and it is still awful. My boyfriend is here alot on the weekends and sometimes she is ok with him and other times she will attack him for no reason. Can someone help? I don't know what to do.

Has she always been aggressive?
Please read this link:
http://www.thecatsite.com/forums/showthread.php?t=20837

Yes she has always been aggressive. I haave read that thread and plan to get started on it ASAP. I have also ordered Pam Johnson Bennett's books. I am at a loss. Again she is usually sweet and loving with me...but a madwoman with others...