Forums mIRC Help mIRC Virus?

I would guess that the reason it's doing it now is because your AV software has auto-updated and the updated version classifies mIRC as a virus. If you look at the ZoneAlarm forum and Kapersky Labs forum (ZoneAlarm uses the Kapersky AV Engine) there are others who have experienced the same issue. I haven't used ZoneAlarm for a few years so I can't tell you exactly how to exclude mIRC being scanned and quarantined but judging by the ZA forum link above it's definitely possible.