Stopping Virus From Coming Through

I wrote befroe but did not get an answer. Please help me to understand. I have windows xp with verizon/msn. my son has to 2 emails and I have 1. I have symantec virus protection and it is working properly. The problem I do not understand is everytime my son signs on to either email address the box from symantec comes up saying 32sasser has been successfully blocked. When I look at my log from symantec it shows it is blocking it constantly.I ran the sasser tool and shows no virus and did sybot and adaware, it showed 265 critical and i removed them. I understand this is being blocked but isn't there a way to stop them or it from trying to do this ?He gets emails showing from postman and RE-- emails and when we look there is nothing on the page.