Check that it's not in the Add/Remove Programs section, see if you can uninstall it from there.
You might want to search for 'mirc' or 'internet relay chat' with
Symantec's search tool, they have a pretty extensive database.
If you didn't install mIRC, and it's starting when Windows starts, I'm 99% sure that it'll be a trojan/virus and nothing else.
Regards,