Any software can be used for malware, from your email software to your web browser. The purpose of anti-virus software is to recognize which software on your computer has been modified in such a way as to make it malware and it does this by monitoring malware distributions around the world and creating signature files that it uses to recognize the malware.
However, when an anti-virus company does not test its signature files well enough, it starts detecting all kinds of software as malware that isn't actually malware. This is called a false-positive detection and results in the anti-virus software deleting perfectly good files on your computer without you being aware of it.
Note that even if you disable your anti-virus or firewall, most anti-virus or firewall software will continue to work in the background and will continue to block applications.
In this particular case, it looks like F-Secure has chosen to blindly mark "mIRC" as malware without actually checking to see whether it has been modified or has other detectable malware files with it.
This issue is definitely a mistake on the part of your anti-virus software and indicates a lack of care and diligence on their part (for a good example, see
this story) and the only solution is to contact them to complain.
Alternatively, you could switch to Eset NOD32, which is my personal choice of anti-virus software. It has one of the lowest false-positive rates combined with one of the highest valid detection rates of any anti-virus software and I would highly recommend it.
