ClamWin uses ClamAV resources but was designed to run on Windows and is somewhat mature. It can work with centralized updates, email notices upon virus detection and runs on any likely version of Windows. It has a plug-in for Outlook and is integrated into Explorer, though I'm not sure it does on-access scanning. (It didn't in the past, but it might now.) We most appreciate it on servers where we have little exposure to viruses but need something that can report if a potential virus is found.

MS Security Essentials isn't for servers, so ClamWin comes in handy where we don't want to spend the money for other AV products. The reporting feature is handy for these machines because many of them don't have someone logging into them through the GUI more than once a month. Be wary of the option to remove viruses from the system rather than just from memory though, in the past it had a pretty high false positive rate. (They seem to have fixed that now, your mileage may vary.)