Personally, I think it unlikely that there are many freshly written viruses around which will attack Win 98 as the rewards will be very few and far between. Indeed the attempts at attacking XP are probably getting less now as the virus writers are concentrating on Vista or Win 7.
I heard of an acquaintance who had a machine he dedicated to internet use only - it ran Win 95 and never had an attack at all! In those days I never needed active virus protection - I just ran housecall every few weeks: it found little and deleted what it found.
P.P.