NIS 2009 scan freezes

Every morning around 5am, I have Norton set up to scan my computer. All of the sudden, when I log on in the morning, it’s still scanning and it reports that it hasn’t even scanned anything. Every other time I attempt to scan my computer, it just sits there at “scanning commonly infected areas start-up files…” without scanning everything. I’m not sure what to do. I am running windows vista home premium SP2…if that helps.