From the help:
A quick scan checks the places on your computer's hard disk that spyware is
most likely to infect. A full scan will check all files on your hard disk
and all currently running programs, but it might cause your computer to run
slowly until the scan is complete. We recommend that you schedule a daily
quick scan. At any time, if you suspect that spyware has infected your
computer, run a full scan.
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As I understand it, the quick scan starts with the content of memory, and
the startup vectors, and works back--so if something is starting from a
drive other than the boot drive, I thiink that stuff "near" that might get
scanned, but not the whole drive.
I generally stick with the recommendation of doing quickscans only. In my
experience, full scans mainly turn up stuff either dead in temporary
internet files, or email attachments sitting in deleted messages or the
like--nothing active. So--I only do full scans when I am feeling paranoid ,
or are doing some other maintenance operation and have time on my hands.
I do full scans slightly more often on servers--to see what the users are
putting in their space.