Using NPE and NBRT the boot.cidox came back. How do you remove it permanently?

Norton Power Eraser (NPE) could not remove the boot.cidox infection.  The Norton Bootable Recovery Tool (NBRT) did remove it but the guy that I helped later emailed me to say it is detected again by Norton 360.  I haven't actually seen the mesage he is getting but it sounds, by his description, to be the same message I saw on his computer when Norton 360 initially detected it after I installed 360.  Does anyone know how to truly eradicate this virus?  Could it be that he is going to a particular web site that gets his computer infected?