First, do a search for "virus" and "trojan" on the forum to see what we have said in the past. That, of course is regarding antivirus apps that aren't mainstream. You shouldn't have the same problem with Norton unless you have a virus definition file that is VERY old.
So... chances are, you have a virus or trojan on your system. Best thing to do is uninstall mirc/Invision (and delete the folder) and then reinstall them. Invision does not have any virii, trojans, or backdoors as long as you get it from an official site and do not catch any from using it online (ie. downloading something you shouldn't).