First of all why would you need RPMs under Knoppix? This is a Debian based distro and tehrefore the packages are called .deb. You can get them with apt-get or, if you prefer with Synaptic (also there's dselect).
Secondly, whay do you need an AV under Linux? You say you didn't need one under Window$ (??!) but you want one for Linux? You're saying that you never had an AV for Win, you browsed the net and you didn't get any virus, spyware, etc. ?