Your computer – how does the virus get there in the first place. SOMEONE must first of all get it to execution. Malware doesn’t suddenly jump in and exist. It has to be brought into the machine. A virus or trojan does jack when it just sits on your machine. It is a program. It has to be executed to do its “magic”.
There are exactly three ways to get this done. First, remote (RPC) exploits, which is easy to defeat with a router that does not allow any packets in to sensitive ports. Second, exploits in programs. This is harder to secure, since you can never know whether your mail client or your web browser (or one of its myriad plugins) has such a vulnerability. Your best bet is to use something that has nearly no market share (and is thus not interesting for commercial malware users).
And finally, the user himself can execute it. And, believe it or not, this is the most used and most successful way of infecting a machine. In other words, the main security problem is not in the machine. It’s in front of it.