Once again … back on topic.
There aren’t any “decent” free anti virus applications out there. True there are loads of freebies but they all seem to lack something or other.
it may be timely and regular virus definition updates; it may be a slow and cumbersome ‘engine’; it may be that they only detect the most obvious viruses; it may be that they have difficulty in removing ‘found’ viruses; it may be a tendency to give ‘false positives’;it may be a combination of some or all of these things.
With anti-virus software, the bottom line is that there’s no such thing as a free lunch.