First,
if you have downloaded your copy of mIRC from
www.mirc.com, then it is fine as downloaded. Of course we cant begin to guess at what else might be on your puter.
Next, there have been cases of what are called "false positives" from some virus scanners. I would suggest contacting the manufacturer of yours (checking their website) to report this ask them what is going on. In the past those involved have issued patches to correct the situation if that is indeed whats going on.