The reason Windows has so many virii is because everyone uses it... If Microsoft falls and linux takes over, you can bet there will be hundreds of linux virii coming out every year just like Windows now. Same thing for Macs...

I dont know about security, but basically, if it's hooked to the net, it can be hacked... no matter how secure.... Although you can tighten up Windows just as secure as Linux. I only have one port open, 21 for my FTP server. Disable netbios as this is a big one... and block all traffic over port 135 (RPC) and disable the windows time service and some others... then place this computer behind a router like the one I use which rejects all incomming connections and only allows outgoing connections through port 80... Pretty secure... from the outside.... fairly secure from the inside where I can keep tabs on the other users...

Seriously though, you want you're data safe? keep it up in your head.