If you are plugged in but never use the web like IE or Firefox, you still can get a malicious software on your computer. Just being plugged into a internet connection puts you at risk even if you never ever touch the computer, so long as the computer is powered on.
The only way to ensure you NEVER get a virus or any other malicious software on you computer is to have no connection to any other computer or device, including a router or internet port on your wall. That even means no cds no thumb drives, nothing.
However i'm guessing that isn't a very good option for you. So download and install Microsoft Security Essentials, and Mal-ware Bytes. Run Mal-Ware Bytes every few weeks or when you remember, and have MSE scan all you drives weekly. This is how i've kept my computer clean for over a year. I've a had maybe 4 items caught and contained by MSE over that time. Mal-ware bytes has not reported a single piece of malicious software in that time. Here's the kicker, i'm on the DMZ, which means no hardware firewall. MSE is hands down the best free anti crap software i've used.