Thanks Flybhx and RadarContact ... I am using only one and have never had a moment's trouble but I have a friend who is using three and whose computer has been badly "virused".
I would imagine that in order to fight a virus, the programme must first be given the information about that virus through updates etc. (my own is updated every four days). If you then have another anti virus programme running, it would not "understand" that this was information only and not the virus itself and therefore try to destroy it. Thus you end up with the programmes attacking each other believing that what they're dealing with is viruses.
Would that be how it works ... or something like that?? I'd like to be able to explain it to some friends without getting technical.
Thanks for the assistance people ... much appreciated.