Antivirus software always warns that any other antivirus currently installed should be uninstalled to avoid serious problems. From what I understand, one program may see another AV as a potential virus, or think that its actions are malicious. What's the actual danger though? Will the two programs try to fight it out?
Specifically, I'm getting this warning from E-Set with Mcafee already installed, though I've seen the same warning from other AV programs.