Glad to hear you solved your problem

As a note of warning, these trojans/virii are happening to so many ppl because they accept/open files from total strangers, or click on urls that infect them. Also, as has been said, many of the standard virus scanners arent as good at catching IRC related nasties as some like the trendmicro housecall ones are, so to be on the safe side, i'd try running one of those as well.
I'm especially glad you now realize the author of mIRC isnt to blame, nor is the real mIRC itself