GPL Antivirus for Windows

Moon Secure Antivirus aims to be the best Free Antivirus for Windows under GPL license. It offers multiple scan engines, Net shield, Firewall, On access, on Exec scanner and rootkits preventions plus features from Commercial Antivirus applications. The installer size is rather small (~11MB) and it installs without a reboot.
Included there is a real-time monitor (RTM), plus an on-demand scanner for drives and a integrated context menu scan for individual files or folders. There is no GUI but you can get a pop-up by right clicking the icon in the system tray where you can initiate updates, various scans, enable/disable RTM or shut-down the antivirus. The updater responds very fast.
The memory usage is a little higher than expected (~100MB for the two processes it runs) and I couldn't find any information about how effective this antivirus is. However I recommend you to download and take a look at it.