Never caught a virus except the winworms

I really don't know how did that happen, but in the last 5 or 6 years i've been using one rather slow pc(cel 400 upgr through time from 64 to 256 ram) and because of that i never liked the result of a antivirus program monitoring my system(and yes, i've been constantly using internet...thru dialup). No program eventually found any virus so after the first year or so i stopped installing any antivirus programs. I didn't have any firewall neither because of the effect of slowing the machine down. And, believe it or not, the first time that virus infected my machine was with the xp worms... applied a patch and that solved the problem, no antivirus program again... what do you think? are those antivirus programs really THAT necessary?