Do you have the latest version of OpenSSL installed? Try installing the latest version to see if it fixes your problem:
http://www.mirc.com/ssl.htmlIf not, you should be able to download the previous version (7.17) here:
http://download.cnet.com/mIRC/3000-2150_4-10001733.html?tag=mncol;1
By the way, I'm running mIRC 7.19 with Windows 7 x64 right now and it's working fine.