Many of my clients have had to contact me in the past week related to new Virus infections.
The axiom: an ounce of prevention is worth a pound of cure is extremely accurate.
Prevention is fairly simple.
Get a reputable anti-virus product installed, keep it updated. Most of the better products ship with software firewalls.
I personally use ESET Smart Security Suite 4 for all of my Windows systems. I use Clamav, Selinux, and IP chains for my servers.
The misconception that an email attachment or an infected file from portable media is the only way to get infected will be the demise of many small and medium sized businesses this year.
AV Comparatives Offers comparative unbiased Anti-Virus reviews.