Can't delete infect file? help please

Um i have windows XP home and installed norton antivirus 2004 on it. for some reason auto protect keeps detecting a downloader.trojan something like that in a file called ms3fs.dll and so i searched the file but it wont let me delete it manually either ive searched bunch of .dll download sites and cant find it need soem help please having this thing pop up eveyr 5 mins is annoying

You could shutdown the pc.......disconnect form the net ......restart pc in safe mode <hold down the f8 on boot> and delete it.....download spysweeper......norton is not that good.....at trojans.........or trojanhunter or swatit