Ok, I just downloaded mIRC from the official mirc download page. Avast says it contains the virus called "idp.alexa.51".

see this thread

I just re-downloaded the installer, and it still has the same sha256 checksum as it had on the day of release.

a248a2104f4965a1926067eb745da1a973696d0dbfb9bfdc0ed674686ef6cc5e

This is called a false-positive and usually happens when an anti-virus company updates their virus definition files without checking them properly. The result is that the anti-virus software starts incorrectly detecting some applications or files as trojans or viruses. Unfortunately this happens all the time. For example, some years ago Microsoft Security Essentials detected the Google Chrome web browser as a trojan and recommended that users delete it.

You would need to contact your anti-virus software company to report the issue and to ask them for a solution. They should then correct the error in the next update of their virus definition files and should be able to tell you how to prevent their software from behaving this way in the meantime.

mIRC and the mIRC installer are code-signed, so you can also confirm that the installer is valid by checking the Digital Signature. This can be done by right-clicking on the installer exe file, selecting Properties in the popup menu, and then selecting the Digital Signatures tab. mIRC is signed twice with SHA1 and SHA256.