re-download the installation program from here
Next run the installation using the options to override all settings. (I don't recall off hand the exact wording)
Be careful that your anti-virus program isn't detecting mIRC.exe as an infected program and auto-deleting it. If you get the installation from the link I provided, then the installation and the files in the installation are guaranteed to be virus free.
Additionally, don't start a second post (either new or as a reply) when you already have an identical topic posted.