My latest research suggests the following numbers
Windows malware : > 150000
Linux : ca. 400 viruses
BSD : 0
There was one virus about 3 years ago that used a hole in the Apache webserver to get into the system. But a) that hole has been fixed, b) normal users do not run the Apache webserver.
So the current status is : BSD IS secure.
There might be some danger if you run Wine as root (=admin) and then manually start a virus (... why would one do that???...). But even then most viruses will just fail after some time and won't do so much damage.
Unlike with Windows, your BSD comes with a stable and long developed Firewall right from the start. (The XP firewall is a joke and nothing else)
PS : A very warm welcome to PC-BSD! Hope you like it!