There is a virusscanner for Linux, ClamAV (
http://www.clamav.net/), but I seriously doubt you're going to need it. To get started, use a user-friendly distribution like Ubuntu (
www.ubuntulinux.org), which is my personal favourite. It'll install in less then an hour and will detect most (if not all) of your hardware (allthough wifi-cards may pose a bit of a problem). You can also try a live-cd first (Ubuntu has one, but Knoppix is also a very good one).
As for the actual using, since most distros startup KDE or Gnome at boot it's pretty much the same as using Windows. There's a big button which gives access to all programs installed... Adding software is mostly done using some sort of package manager (Synaptic in Ubuntu) and is really simple and straightforward...