i can't change the progamme msn messenger uses to scan viruses

hi, i was wondering if you could help me please, my problem is that msn
messenger scans files i am sent over msn with Microsoft Windows OneScanner
but that programme is no longer on my computer, i am now using Norton Anti
virus. I have tried to update these changes but i can't because the browse
buttons in the File transfer option are inactive. I have also tried
re-installing msn messenger but that didnt work either. Is there anway of
changing the virus scanner through the registry because i have found the key
where it lists your virus scanner but im not sure how what to edit the
details of my virus scanner. If you can help me thanks in advance :-).

BTW: im using msn messenger 7.5