Does Microsoft’s Windows Defender do anything that Avast does not?
The marketing words are that Windows Defender is for “spyware and malware” and Avast is an “anti-virus”, but these terms are used very loosely nowadays.
Is it known if Windows Defender detects anything that Avast does not?
Is it known if Windows Defender detects anything that Avast does not?No it is the other way around ;)
Your best combo is avast + malwarebytes + MCShield
There is no such things as “only antivirus” anymore. Term remained, but all the “antivirus” programs evolved into “antimalware” years ago. avast! is no exception.
What does Windows Defender do more than avast! ? It has a simple interface with minimal features. Sometimes that can be a positive thing. But in any other aspect, avast is light years ahead of Windows Defender…
To quote David Lippman at Malwarebytes forum
All virus are malware, but all malware are not virus 
When I mentioned the marketing words, I underestimated the savvy of the audience here. I was trying to avoid people replying that Windows Defender is just for “malware and spyware” and Avast is just for “anti-virus”, and therefore there must be differences.
So let’s just ignore those marketing terms completely, and get back to the basic question: Does Windows Defender do or detect anything that Avast does not?
No!
Avast does things which Windows Defender does not do…offer real protection!
You may find this interesting http://www.howtogeek.com/173291/goodbye-microsoft-security-essentials-microsoft-now-recommends-you-use-a-third-party-antivirus/
In other words Microsoft recommends using a third party anti-virus.
MalwareBytes Anti-Malware is another excellent addition to the toolbox.
https://www.malwarebytes.org/
It was answered in first reply
You may take a look at http://www.av-comparatives.org / http://www.av-test.org/en/
One big difference:
http://www.screencast-o-matic.com/screenshots/u/Lh/1412933687695-75098.png
Check and see how many you receive from Security Essentials (Defender in Windows 8 and up.)
Win defender used to be updated once a month, it came with the monthly win update
Dont know if this is changed in later versions …
Its now updated regularely, sometimes a few times a day, and can also update via a Built in updater, no longer Windows update need.
Besides speculation, does anyone have any real documentation whether or not the coverage offered by Avast anti-virus is a true superset of that offered by Microsoft’s Windows Defender?
Specifically, are there any types or classes of malware from which Windows Defender does a better job offering protection?
Can someone who works at Avast provide reliable answers to these questions? Your answers will be greatly appreciated! 
See the tests. Windows Defender is always at the bottom and is always taken as a minimum. avast!, while not top performer with version 2014, it certainly always outdoes Windows Defender…
Right, when third-parties test for detection of common viruses, Avast typically detects a higher percentage (but not all) than Windows Defender.
But given that Windows Defender and Avast Antivirus are marketed as very different types of programs (Windows Defender is marketed as “spyware and malware” detection and Avast Antivirus is marketed as “anti-virus” protection), I am wondering if there are certain types of malware for which Windows Defender provides better protection or detection than Avast Antivirus.
I’ve explained the “antivirus” terminology before. There is no such thing as “antivirus only” anymore. In fact, there are hardly any actual viruses (file infectors) around. What you’ll see in 99% of times are bots, backdoors, spam bots, trojans and bunch of spyware crap. One of rare file infectors lately were Virut and Sality. But these are in real minority these days though still very annoying to clean if infected. That’s why name “antivirus” remained with some programs even though they detect all sorts of malware and are in fact anti-malware programs. Avast! is no exception. They just kept the “antivirus” in the name instead of renaming it to avast! Anti-Malware. Just name and nothing else.
Avast is Antivirus - Antispyware, complete Antimalware protection, it will detect everything that Defender can and much more so you can safely disable Defender as it’s not needed and is just wasting resources.
Note that Defender on Win8 systems is Antivirus - Antispyware and will disable itself when another AV is installed.
Windows Defender starting in Windows 8 became your all in one AV and Malware program.
I didn’t say how good a tool it is but simply made you aware of the fact that you need to know
which OS you’re using in order to determine what Defender actually is.