Are you using anti-virus or firewall software? Some anti-virus or firewall application may incorrectly think mIRC is a trojan/virus and can prevent it from running correctly. You should first update your anti-virus/firewall software. If that does not help, you would need to add mIRC to the exceptions list of the anti-virus/firewall software.
Also, please type the following into mIRC in the status window, press the enter key, and report the results here:
//echo -a $os $version $beta $md5($mircexe,2) $file($mircexe).sig $script(0) $dll(0) $com(0)