Without telling it what to scan and when,
ClamXav by itself doesn't scan anything, let alone automatically. The prefs provide an option to schedule items (files, folders) to be scanned. The only auto-scan you can set in the General prefs pane is that of email messages and mailboxes, provided the latter are of the mbox type. You will still have to tell ClamXav
where to find your email folder for these scans to actually be carried out. The prefs also let you check for and install updates automatically upon launch.
The ClamXav
Sentry function (to be activated separately) allows you to select folders to be scanned when changes (read: additions) to them occur. You may want to consider activating Sentry for your downloads folder, and any other folder that receives files from (uncontrolled) sources other than your computer. To ensure that Sentry always runs it has to be added to your Login Items. Scanning your Downloads folder will take CPU cycles during downloads, and should this prove too intrusive, you may choose to temporarily suspend scanning or to scan at other times.
