MS Windows (native): A native port of libclamav, clamscan, freshclam and sigtool is being developed by Bransoft at http://www.bransoft.com/clamav.html .
Another native port is being developed by NetFarm? (http://oss.netfarm.it/clamav/) which recently joined their forces with ClamWin (http://www.clamwin.net) . A package including a native port of ClamAV with a nice GUI written in C++ and Python is available at http://www.clamwin.net. ClamWin allows to select and scan a folder or file, configure settings and update virus databases. It also features a Windows Taskbar tray icon and a context menu handler for Windows Explorer which installs Scan into the right-click explorer menu for files and folders.
MS Windows (Cygwin): ClamAV is a part of the official Cygwin port repository.