Norton Power Eraser -- It Works

One of my computers has been behaving badly for about a month.  I'd run the regular Norton but nothing was found.   Then I ran the Norton Power Eraser and it found a boot-sector virus and removed it. Since then, the computer has been running like it's new.  But, I have a question.  Why isn't the capability of Norton Power Eraser included in the regular Norton anti-virus product?  I'd think that the regular product would remove everything and not have to go to a special product.