A lot of uninstall issues are covered in this thread
, although most of it relates to security issues. It's worth noting, just because you installed mIRC intentionally doesn't mean you are not infected with a trojan - a trojan may have hid mIRC at a later date from the Add/Remove programs list. There are numerous antivirus/firewall resources in this thread
Make sure you only get mIRC from this website, and try what Danman said.