Windows Defender detected a virus on my computer and quarantined it. Is it better to leave it as it is or to delete it?
In general, what are the circumstances under which one must not delete quaratined files?
Delete it unless you think it is something new and unique that an anti-virus company might be interested in (chances are it is not).
I would not delete a quarantined file if it is a program you use all of the time that has been flagged incorrectly by anti-virus software. It is rare but it has happened. In that case you would want to wait until the anti-virus software is updated then release the file from quarantine.
Meanwhile the choice to delete or not to delete the files in quarantine is a doubt for Hamlet and can be considered opinion based; really it depends from many factors as the space availability, which kind of file they are, the reason of quarantine, the action of the virus (if it overwrites the file or shifts the data), the possibility to find a copy not corrupted of that file...
Some general considerations (so far to contemplate them all):
Usually a smart antivirus knows when to permanently delete a files because it will be not possible to save it, and when there is enough hope that a patch can arrive before or after...
it's for this they did the quarantine :-)
You can search info about a file on your preferred search engine, you can ask to sites as Computer Hope , or use tools to repair missing or corrupted system files as the System File Checker .
You can always manually replace a corrupted system file with a known good copy of the file, maybe taking it from another computer that is running the same version of Windows of yours...
md5sum or other checksum can guarantee that the files is a good copy.
Collecting information about your virus will grant you a wider and wiser choice of possible actions.