that is because it picks a random email to put 
into the "from" category.  Say your computer is 
infected...it will take an email from your list of 
emails and send the virus out using that 
person's email in the "from" category.  so if i 
get an email with a virus with the "from" 
category being your email, i know it isn't you 
who sent it....i also can't assume your 
computer is infected (though i can't assume it 
isn't).  

mb