there are many trojans that drop an old version of mIRC as part of their payload, sounds like you got one. A normally installed mIRC would be easily uninstalled. Run a good virus scan, like
this one which is pretty good at catching a bit more than others. Note the name of whatever is found, since you may need to do some additional editing beyond what they can remove. Use the search engine here, all forums, all posts with the word trojan and you will see a lot of threads on this.