All mIRC installation files on the mIRC website and mirrors are checked for viruses before being made available to users for download, so it is not possible to get a virus from them.
The anti-virus software you are using has made a mistake, and thinks that the files have a virus when they don't; this is called a false-positive, and happens after an anti-virus company updates it's virus definition files without checking them properly.
You should email the anti-virus company to report a problem with their software.
It looks like Sophos Anti-Virus didn't properly research this virus, and their new anti-virus update is now detecting all installed copies of mIRC 6.03 as a virus.