It is almost certainly a trojan. Go here
and run a free online virus scan, which will probably work better than Norton or MacAfee (or whatever you're using). You should then uninstall mIRC and then do a clean
re-install. Be wary of clicking on links that are spammed to you on IRC. Even if it's from someone you know, they could be infected and not realise it. Go here
for more links to security and virii information, and here
for more information on trojans. Hope this helps!