without knowing what error msg you are getting ( or for that matter what exactly you downloaded), its almost impossible to help you.
mIRC is shareware, and has a 30 day evaluation period and is easily uninstalled. As phoenix suggested, if you didnt get whatever you downloaded from an official www.mirc.com
download site then you may have gotten a trojan. Try doing a couple good virus scans to see whats found. Theres a good free online one at http://housecall.trendmicro.com