It was probably an issue with the antivirus, or a security setting that got messed up, or just a corrupted file. Keep in mind that even if the antivirus/firewall is set to allow a program to run, it may still quarantine the file if it believes it's a virus, or in some cases (such as McAfee) a "potentially unwanted program" without telling you. That's usually why you get that error message when you try running it. By installing 6.35, it will place the mirc.exe back where it's supposed to be and it should work fine unless the antivirus quarantines or blocks it again.
If you keep having that problem, a good idea is to notify the antivirus company that the file isn't a virus so they can remove it from their signature files so it isn't affected any more.
Also, many antivirus programs allow you to whitelist programs, files, or folders. That will prevent the antivirus from scanning or checking what you add to the whitelist. You could add mIRC there if you want and it would solve the problem for you. Just be aware that if some virus got into a file/folder that you add to your whitelist, your antivirus would never know, so be careful with what you add to that list.
