Norton 360 shows Trojan.Brisv.A!inf, how do I get rid of it? HELP ASAP!! ?

I downloaded the repair/removal from symantac and it will not let me run it~says I do not have network administrator permission. I am the owner of my computer so why do I need permission? How do I remove this virus before it kills my computer? I tried norton chat help and kept getting put on hold and never helped. I downloaded an mps file from limewire.com. I have Norton 360 WHY did I get this virus if I have current virus protection? I need to get rid of it....PLEASE HELP?