I use a remote SME server running Affa to backup a Windows 2003 server.
Rough details:
1) At midnight each night I create shadow copies of C: and D: on the Windows server exposed to the network as "\\servername\shadowc" and \\servername\shadowd"
2) I have VMWare and a small Ubuntu virtual server installed on the same box. The Ubuntu VM uses 128M RAM and 1GB hdd.
3) I have Affa on my remote SME server connect to the Ubuntu machine, then backup \\servername\shadowc and \\servername\shadowd
Problems / Notes:
This method gets all of the data and files on my server, but none of the access control information (user and group rights, network share information, etc)
Michael Weinberger has described a potentially better way to do this by installing an rsync server daemon directly on the Windows box, instead of my kludgy virtual ubuntu method...
If the SME server were local to the Windows box you would not need the VM, nor would you need to install rsyncd - you could use SME to backup ...\shadowc and ...\shadowd directly.