Yes, the quick scan is very useful. It looks at all of the actively running
processes on the machine, as well as files that are pointed to by the
various registry entries and other locations used by spyware to start
automatically or otherwise interfere with your system.
If you have a particular piece of spyware or other unwanted software that is
detected by Windows Defender, the vast majority of the time it should be
detected by the quick scan.
The full scan, in addition to performing all of the checks in a quick scan,
also examines every single file on your system. This is a quite
time-consuming operation if you have a lot of files.
I would always start with a quick scan - if it gives you a clean bill of
health, you can be pretty confident in the results. If the quick scan finds
anything, you probably should follow it up with a full scan to find any
additional junk that the malicious software dropped on your machine that may
have been missed by the quick scan. It's also a good idea to schedule the
full scan to happen periodically (every night, for instance) at a time when
you are not typically using your computer and the added time to scan all of
the files won't be noticed.
-Mike
