Not sure what everyone is using for Windows Antivirus, but Microsoft Security Essentials is close to releasing version 2.0. More than likely when you bought your computer there was some sort of trial Antivirus loaded that you may or may not have purchased. Many ISPs provide Norton or McAfee for free as part of your internet package. One of the great things about Microsoft's Security Essentials is that it provides malware protection as well. "Instead of relying on a virus database for detections, MSE 2.0 has a heuristic engine that can detect previously undocumented malware, so you're safer against new variants. It's worth noting that this is often a double-edged sword, as heuristic technologies can also increase the likelihood of false positives.
MSE 2.0's network inspection feature is as straightforward as it sounds: it watches for suspicious traffic and protects your system from network-based attacks. Unfortunately, this isn't available for Windows XP users as it requires the Windows Filtering Platform in Windows Vista and Windows 7." Source: http://www.techspot.com/news/41638-mse-20-released-with-heuristics-network-inspection.html
If you insist on paying for Antivirus software I suggest staying away from all things Norton and McAfee. Try Sophos, or Comodo which have been repeatedly recommended by security penetration test firms. They don't waste money in advertising dollars hiring people like Snoop Dogg and David Hasselhoff.