I run Mirc and irc program daily - for several months as well as Avast! and never had a conflict before
Today avast updated its data base (May 22, 2006) and suddenly Mirc.exe is being blocked and called a trojan.
I believe it is a false postitive and you need to check your data base.
I let avast! clear it out and reinstalled a new version of mirc after scanning and finding no other traces of viris on this computer - just to be on the safe side.
Well the brand new install - was also having the mirc.exe identified as a trojan and refused to allow it to run.
