I am running Arch Linux and the Anti-Virus available by Pacman was "ClamAV for Unix-like systems" which I'm assuming would not search for windows viruses.
You shouldn't "assume" anything
scans for windows
viruses. It may also scan for linux viruses (I don't know), but I don't think there are enough of these to pose a serious threat.
I use linux only, and have never used a virus scanner. No problems so far.
[Edit: Camorri beat me to it! /Edit]