I would recommend the following strategy to keep yourselves virus and spyware free.
1. Install and run Antivirus on all machines connected to your network.
2. Use the free online antivirus tools to check your main machines once a week. You can find them here;-
Use all options when scanning and choose to remove viruses if any are found. Why use all 3? Because, I have found that each antivirus no matter how good they are misses viruses so using your main antivirus solution on a daily basis and the online ones on a weekly basis you will keep up to date and virus free.
3. Install and use Spybot S&D on all machines as above to check for and remove spyware. Spyware can be just as malicious as viruses and the combination of both technologies is the only way to stay safe.
4. Keep the PC’s patched using the Microsoft update service.
5. Make sure you have a good backup regime as no protection employed can keep you safe from the unknown virus that wipes your system.