mIRC doesnt act that way, sounds like you got one of the many trojans that drop an older version of mIRC as part of their payload. Run a good virus scan, and try this online one as it tends to catch more IRC type things than some others do, at this
web page Make a note of what it finds, and look into it since some will require a bit of registry editing they are unable to do for you. Use the search feature here, all forums, all posts with trojan as the subject and you will see a lot of info.