People mistakenly think that their security is as good as their security habits .... a bad assumption. Your security is only as good as the habits of every web site manager at the sites you download files from and every person who sends you attachments. I have found infected file in e-mail attachments, in free as well as commercial software upgrades and in on-line file libraries.... and yes trusted sites. When I served as a Compuserve Wizop, we oft had files pass a virus scan by one program only to be caught by another hence the double scan rule.
Microsoft Defender and SE provide a bare minimum of protection. While when I am asked to clean a machine, I see many levels of infection, the record was over 1200 and this was with a machine using MS protection. The machine belonged to a teacher whose students submitted homework by e-mail.
http
/www.av-comparatives.org/summary-reports/
"The following vendors participated in AV-Comparatives’ public main test-series of 2013 and agreed to have the effectiveness of their products independently evaluated. Microsoft ***asked*** to be included in the tests, but to be regarded as a ***baseline*** and not as competition to the other products. "
If you think software can't detect 0 day attacks, read up about hueristic scanning and 0 day is not 0 day anymore the third day it's been out .... the better vendors usually have signatures for these by then, others don't....any every vendor gets caught short every now and then.
The most common infections I am asked to clean come from peeps who clicked on a "Free AV scan" or "download free AV software" link.