There is no chance that the mIRC installer on mirc.com is infected with any virus trojan or anything of the sort. Now, however... Many programs will mark mIRC as such because they do not have a correct listing for the program and since it has been used to create and distribute such malicious code, programs will mark a clean installer as such. You can either try to get the makers of the program that marked mIRC as bad to update their definitions or you can just go on and use mIRC.
edit:
http://virusscan.jotti.org
File: mirc635.exe
Status: OK(Note: file has been scanned before. Therefore, this file's scan results will not be stored in the database)
MD5: ce01307967773104627ec80e44e60e88
Packers detected: -
Scanner results Scan taken on 14 Jan 2009 14:30:18 (GMT)
A-Squared Found nothing
AntiVir Found nothing
ArcaVir Found nothing
Avast Found nothing
AVG Antivirus Found nothing
BitDefender Found nothing
ClamAV Found nothing
CPsecure Found nothing
Dr.Web Found nothing
F-Prot Antivirus Found nothing
F-Secure Anti-Virus Found nothing
G DATA Found nothing
Ikarus Found nothing
Kaspersky Anti-Virus Found nothing
NOD32 Found nothing
Norman Virus Control Found nothing
Panda Antivirus Found nothing
Sophos Antivirus Found nothing
VirusBuster Found nothing
VBA32 Found nothing
