mostly i have found its only norton that gets upset (thers another post in software where i have a big go at norton incase you are interested btw lol). if you use more than one, especialy those that are made different people, one may find some things that the other didnt pick up on, however having said this, depending on the program, they might scan the system in a way that requires it to be the only antivirus program on the system, so basicaly its down to the individual program; no easy answer sorry...