Antispam integration with multiple email addresses in Outlook 2003

I've been a regular NIS user for many years.  I've only just recently  noticed that some of the email accounts I check with outlook are not protected by Antispam.  I've sought out settings to protect more acounts but I dont even see how it is protecting the main address I use.  I came to find this situation by having advertisments delivered to my young boy's email address specifically reserved for chatting with grandma.  Appreciate a point in the right direction, thanks.  I'm a WinXp Media Center edition  user on SP3.

 

M.