The only place that will give you a guaranteed safe and un-modified copy of mIRC is from
http://www.mirc.com/get.html.
If you get mIRC from any other location, and there are lots of them out there, you risk getting a version that contains unwanted scripts, viruses, trojans, adware, and/or spyware (etc.)
If you didn't get your copy from the above location, I recommend:
1) Uninstall your current version
2) Download from the above link
3) Install the newly downloaded version using the Update option.
3a) If the installation does not complete, or mIRC fails to run after installation, temporarily disable your anti-virus program and re-install.