Primarily most virus/spyware/trojan is windows based, as that is the biggest population of operating systems out right now. In addition to that, linux has better security because your user does not run as root normally.

Currently I run a hardware firewall(router w/NAT) and iptables (software based firewall), I also perform regular checking with some security tools since I am running a couple of servers.

Check out https://help.ubuntu.com/community/Security for information related to security and linux/ubuntu.

Have a good one.