1) Which mirror did you download from?
- There is next to NO chance that any of the download mirrors on www.mirc.com/get.html
(the ONLY official place to download from) contain a trojaned mIRC client. That is the ONLY place you should download mIRC from - and the latest version is 6.14.
2) Clean your system of all trojans and viruses properly, reboot, install mIRC 6.14 and then connect. See if the same thing happens. See this post
and this page
for some virus assistance.
Hope this helps and stops this strange behaviour