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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?

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    I'd just like to say that you should probably be using a real antivirus like avast if you're worried about viruses. – Hellreaver Jan 25 '16 at 17:16
  • @Hellreaver Why, is Windows Defender unreliable? I'm not worried much, it's only that I often find my pen drives to be infected. – ghosts_in_the_code Jan 26 '16 at 8:34
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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.

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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):

  • If the file are not important (opinion based) you can delete them.
  • If you can find a copy not corrupted of the same file, (maybe from another clean computer or from a backup) you can put it back and delete the file from the quarantine.
  • If there are more than one copy of the same file attacked by the virus in different ways you can keep them waiting for a patch that will be able to fix the problem.
  • If the virus is new and it can be studied, you can send it to the antivirus software house.
  • If the virus do not overwrite the file but scrambles it, you can decide to wait that a patch will exit (In the hope that you will see that day).
  • If the file was a part of a program (and not a file you created) and you decide to reinstall the program, you can delete it.
  • If the file is a fake dll, or a file created by the virus, you can delete it.
  • If the file is a file created by the virus to replace an existing one, search for the original one and delete the quarantine copy.
  • If you have space on the HDD you can always decide to keep them, till they remain in quarantine they are harmless.

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 [1], or use tools to repair missing or corrupted system files as the System File Checker [2].
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.

Note that there exist many patches virus specific that you can download from the major antivirus software houses [3],[4] ...

Collecting information about your virus will grant you a wider and wiser choice of possible actions.

  • The files were located in system32 folder. How do I know if they are essential files, or ones created by the virus? – ghosts_in_the_code Jan 26 '16 at 6:44
  • "Many of the system files Windows uses to run are stored in this (system32) directory." Each time is a different case, you can search on internet or on the Computer Hope site for that file name, then on the Microsoft site to understand better. If you can find on the Microsoft Site (look well for the version, subversion, date, size...) maybe you can download it too (and so you are free to delete from quarantine). – Hastur Jan 26 '16 at 7:23

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