The official version of mIRC can be un-installed via the Windows Control Panel Add/Remove section, the same as with all Windows applications.
However, if you have found mIRC on your computer and you have not yourself visited the mIRC website at
http://www.mirc.com and downloaded mIRC, this means that you have downloaded a trojan/virus from some other website or through an email, and this trojan/virus has itself installed a non-official copy of mIRC on your computer.
The trojan/virus you have downloaded will most likely prevent you from removing mIRC from your computer. Even if you were to find a way to remove mIRC, the trojan/virus itself would still remain. You must ensure that you are removing the trojan/virus itself, otherwise your computer will still be at risk.
The only way to remove a trojan/virus is to use anti-virus software that recognizes the trojan/virus that you have. If your anti-virus software does not work, you will need to contact an anti-virus company to ask for help.
You should always ensure that you are up-to-date with the latest Microsoft updates for your version of Windows, as well as any updates for your web browser and anti-virus software.