First off I'd check your mIRC installer's CRC is
2AFD6A5E, if it's not re-download. There was a bad mirror up around a month ago which you might have downloaded from. Whether that was right or not I'd run a virus/trojan scan using some of the more
IRC-centric scanners (Norton, McAfee, etc. can be really crap at finding IRC-related stuff sometimes). If none of that comes up with anything then I'm out of ideas unless you happen to know of something that's changed in your connection (ISP, networks, routers, firewalls, etc.)