I use Avira AntiVir (Free Version) on Windows XP and it works fine, but I can't figure out how to tell it to only check drive C.

I recently installed a Western Digital Passport USB backup drive and noticed that Avira is now scanning that as well. I can’t figure out how to get Avira to stop doing that. Any ideas? I might occasionally want to scan the Passport but certainly not every time. I can’t find any drive specifier in the config screens.